OK so I am about to distill a ton of research and experimentation into a very simple process you can follow to create your own custom curve. I am not going to go into the details of where to get the tools and setting them up - that's been covered elsewhere. Also this is not meant to be "all encompassing" so you may need to do more research to understand some of what I'm talking about here. Rather, the goal is to give you the quick steps to get your own curve up and running. So here ya go:
[Note that much of this is from memory - I need to revisit this and double check a few things, so for now consider this a draft]
1. Start with the latest version of Arve's tool from the wip branch.
2. Select a custom gamma slot, such as Custom 1, in the JVC. Change the correction value to "Import"
3. Set your contrast and brightness to 0 in the JVC and in your sources such as Panasonic UB900. This assumes Standard HDMI type.
4. Run Arve's tool menu.py.
5. Select lp, then option 2. This loads a default HDR curve into the tool.
6. Make sure your source is outputting HDR and not SDR BT2020.
7. Bring up a HDR image you want to test with. I use the "sky scene" in Pan at 18:55. However this scene only resolves up to 2750 nits. Although this is a good choice for those with large screens and low peak white nits (below 100, I only have about 45), if you have above 100 nits you'll be better off using a scene that resolves up to 4000 nits. Mad Max is often mentioned for this. You can find examples of scenes in a picture comparison thread several pages back. Note that I recommend using an actual scene (paused) from a HDR movie for this and not a white clipping pattern. The reason is because the clipping pattern can still show bars but crush in between. The best way to determine this is therefore with an actual scene. Certainly you can try it with a clipping pattern if you'd like to compare the results to doing the same approach with a scene from a movie, but if you only want to do it once, use a movie scene not a pattern for this.
8. Enter ga in the tool (gamma adjust). This will bring up a new menu.
9. Use option 3 "tune with contrast". This will load the tool's default HDR curve into the JVC and prepare itself for your contrast control adjustment (see next step).
10. Your image will be highly crushed in this initial state. See attached pan-sky-initial image. Lower contrast to the point where the crush is gone. See pan-sky-contrast-fully-tuned image - this is how mine looks once fully adjusted for no crush. Just keep dropping it until you feel that no image detail is being lost due to "bloom". When you find that point, raise contrast again a click or two and confirm that the blooming starts, lower it again - the point is toe just confirm that you have the right point. In my setup I think I had to drop contrast to around -18 or -20.
11. When you are done adjusting contrast, press Enter on Arve's tool (which is waiting for you to do so to tell it you're done adjusting contrast). This is where the magic happens... It will readjust your curve based on the brightness you need, send the new curve to the pj, and then automatically reset the JVC brightness back to 0 for you.
12. It gives you a chance to repeat this process, which may be necessary to fine tune a click or two. Change contrast a few clicks in either direction. Most likely you will be good now at 0 if you got it right in the first place (step 10). If you adjust contrast, go to step 11 and repeat. If you leave contrast at 0, when you hit return in the tool it'll exit this adjustment phase.
Tip: From early experimentation it seems that you can get extra brightness without as much crushing on 1000 nit master titles. Therefore if you have low nits like me, it may be worthwhile to have two "go to" curves - one for 1000 nit titles and one for 4000 nit titles. So repeat this whole process (all steps here) a 2nd time, but using a 1000 nit title like Lucy. The result will be a brighter curve that may work well on 1000 nit titles.
As a general guide, expect to lower contrast to about -17 to -23 for 4000 nit titles (crushing beyond 2750) and to about -5 to -7 for 1000 nit titles. Let your eyes guide you on the scenes, but that should give you a rough idea if you are in the right ballpark.
Tip When using the contrast control to set the contrast so there is no clipping, you will likely create a picture that looks nice and properly saturated, but is too dim, particularly if you have less than 100 nits to start with. Therefore I recommend doing one curve with contrast set ideally, again, likely you will find that be around -23 to -17. After completing all the steps listed in this guide, repeat it from scratch again, but this time purposely crush some of the scene, perhaps with a contrast setting around -12 to -15. Finish all the steps here again, but save this into another preset. Then repeat everything once again, but with targeting a 1000 nit master and clipping around 1000, which will be around -5 to -7 as mentioned above. Save that into the 3rd slot. Then as you watch movies play around and see which one you will like best. Hint/spoiler alert: You will like the brightest one best.
However you will crush some bright scenes as a result. So you'll have to decide what is an acceptable level of tradeoff. By having 3 different curves you can try any of them A/B to pick your favorite. You can even use the 1000 nit target curve for 4000 nit masters (but will crush them quite a bit) and still find it VERY pleasing, if you don't mind the crush.
13. At this point your curve is mainly complete. However this is where manually tweaking can be done. You can try changing the soft clip value (sc) lower or higher, and or changing the se value to control how steep the clip is. I lowered my sc from the auto setting and lower my se from 0.75 to 0.5. This further helped to tame some minor blooming I had in the highlights otherwise. For instance, after my contrast was tuned I still had some pretty heavy blooming in highlights. See pan-hand-highlight-high image. After lowering my sc and se I was able to tame it, without sacrificing much if any overall image brightness. See pan-hand-highlight-medium for the end result. Look at Pan's hand (wrist really) in both of those pics. Notice the bloom is tamed (somewhat) in the medium one. Look at the clouds above his hand in both pics - you can see how the excess bloom is tamed there as well.
Note: You will have to walk a fine line between how much crush you are willing to take for a brighter picture, or put another way, how dim of a picture you are willing to accept to minimize crush. The brighter the picture, not only is there more crush, but the picture begins to desaturate. In these two sky image examples those are at both ends - too high and too low. This picture is rather dim in the low one. So in reality I wind up cheating-up some by introducing some desaturation and some crush, but no where close to what the initial sky looks like. I have a few versions of this saved into different presets so I can change the curve to taste based on the movie.
14. Now it's time to fine tune the Brightness, tho we will not do that with any controls - just with Arve's tool. Leave brightness at 0 in JVC and elsewhere. Don't touch those controls... Bring up Ray's black clipping pattern, the 2nd one with the large bars. Pause it with the bars showing so the clip doesn't end on you while you are working on it, but hit play to help you see what's flashing or not as you make adjustments, then rewind to the beginning of the clip so it doesn't end on you as you work with hit. The idea here is to purposely crush black so that 0.005 nits is the black floor instead of 0. This should provide a lower black floor for content mastered at 0.005 black (about 70% of the content) while also not having any detrimental impact to content mastered at 0 nit black. To do this we target bars 77 and below to not be flashing, and 81 and above to be flashing.
15. Enter the command "bbi 0.005" to set this as your black floor for content. Issue Pw command to write this to the JVC. You will likely find that bar 81 and possibly higher bars are not visible/flashing. In that case use the "bbo" command to add just a touch of brightness, then send to the JVC with Pw command. Try something very small at first, like 0.002. You want to get to the point where you can't see bar 77 but can see bar 81. Keep trying bbo values and Pw until bar 81 shows but bar 77 does not.
16. At this point you are done. Use the command "s filename" to save your curve to your PC. Later you can use "lf filename" to reload your curve back into the tool and write it back to the pj, in case you want to work on it some more (or in case you want to overwrite it temporarily in the pj but then come back to it later).
17. Experiment. Try the same process over, but perhaps crushing your contrast-setting scene a bit more to give you more brightness. And or play with sc or se. Write curves into other custom gamma slots. Then bring up scenes and switch between them in the JVC. You may find that some curves work better for dark vs brighter movies.
Hope this helps!
Edit 3/14/17: Added screenshots and further clarification to steps 9-13.
Edit 3/15/17: Added more details on using bbi and bbo to set the proper black level; added more details on contrast fine tuning and idea of designing curves for 1000 vs 4000 nit titles.
Edit: 3/16/17: Added clarification to say to use a movie scene for the contrast tuning, not a white clipping pattern.
Edit: 3/17/17: Added tip above, for ideas on how to generate curves with different brightness levels to pick from.