OK, let us break it down point by point:
Original inquiry - would it make sense to use a higher backlight setting to calibrate lower blacks? YES if backlight is linear. NO if backlight is not linear. We already established that it was not linear before he posted his reply, so using it did not make sense. It would make sense if it were linear because it would be easier for my colorimeter to read low black levels more accurately.
Backlights aren't always linear - though they should be. Variations tend to appear at the lower few adjustment steps and the top few adjustment steps. If you are somewhere in the middle of the range the backlight tends to be the most linear.
Yes, I learned that by running grayscale at different backlight settings and I posted that my backlight was not linear - no reason to repeat that.
But that doesn't really matter... blacks that aren't black are a HUGE distraction and the sign of a poor quality video display. Your number 1 concerns should be getting the black level as low as possible while still achieving a satisfying luminance level for 100% white (somewhere in the 30-40 fL range for a dark room, brighter for a room with some light or a lot of light).
Really? How many posts of mine did he read and how many times did I say what my black levels are and how my TV is calibrated and has been for a while now? Many! If he even remotely followed any threads I made - he wouldn't be telling me this because my TV was already calibrated properly. OK, its not his job to remember my threads, but my oh my - what is that under all of my posts? Is that a signature??? Yes it is! Does it show my black point and white point? Yes it does! Does it say "calibrated" ??? YES! The only uncalibrated part are the low black levels, which are adjusted by a LUT, a software LUT. Software LUTs read the original low blacks and make adjustments on top of any TV controls. I use BT.1886 gamma and for that a software LUT must adjust my lower blacks. It doesn't do a good job because i1Display Pro does NOT read such low blacks WB accurately, which is why it is advised to use your eyes to adjust blacks with TV controls, but I obviously cannot do that if I want BT.1886 gamma. I do not need to provide all these explanations to anyone - I just wanted an answer to my question, which he did not provide and went into another direction. It should be obvious by now that I asked the original question so that I could correct that tiny inaccuracy in black WB by increasing my backlight to make it easier for i1Display Pro to read blacks accurately, running a LUT calibration, and then decreasing backlight back to where it was before. However, that would only be possible if my backlight was linear, but it isn't.
If the backlight gets non linear at low settings that give the best blacks, you may still be able to get a good calibration at the low setting if the calibration controls have the range to fix the non-linearitiy issues. For viewing in a brighter environment, you'll likely need to increase the backlight setting so it's probably best to calibrate some other picture mode for bright-room viewing (again, this assumes the backlight is non-linear and that raising it will whack your low-setting calibration pretty significantly). If you are lucky, the TV will let you save different calibration settings for a brighter mode along with a higher backlight setting. If you are NOT lucky, it will be more difficult to deal with getting the blackest-blacks
My TV is in a room without windows where its pitch-black even during daytime. Nothing in those statements applies to my case. Again, this TV is already calibrated the best it can be, given the tools I have.
You are still overly obsessed with contrast ratio. You should be obsessed with getting the black level as dark as possible and the 100% white luminance level up to a comfortable viewing level. If you want a high contrast ratio, keep in mind that a TINY improvement in black level can make a huge difference in contrast ratio... for example... dropping the black level from 0.06 cd.m2 to 0.03 cd/m2 will double the contrast ratio for a 0.03 change in black level. To make that much difference in contrast ratio by making the picture brighter you'd have to go from 100 cd/m2 to 200 cd/m2.
Obsessing with CR is what got me from black level 0.06 cd/m^2 and 1700:1 CR to black level 0.04 cd/m^2 with 2805:1 CR. If I would've listened to his advice on not using LUTs and PC graphics card - i would've stayed with CR 1700:1 because my TV controls are broken when it comes to contrast adjustment. Only a software LUT could fix it and it did. Again, this TV is already calibrated besides a slight inaccuracy in black level WB.
But there are lots of reasons you may not be able to use the lowest available backlight setting... severe color shifts you can't fix would be one of them. Not being able to get 100% white bright enough to make the images comfortable for the ambient light would be another reason. But if you can get images you like between say, 5 and 15 (backlight setting), using the 5 setting will make the black level the lowest, so that should be your priority backlight setting for calibrating the display for dark room viewing.
Again, this TV is already calibrated at a luminance level of my liking. I use 3DLUTs so the end-result is near-perfect. I just wanted to fix WB of lower blacks... Again, I do not need to adjust my TV to any ambient light.
When there's light in the room, the light de-calibrates the image a fair bit AND if the room has daylight coming in, the color of the light changes minute to minute... different at 8am, 10am, noon, 2pm, 4pm, etc. Different if you have leaves on nearby trees (summer) or not (winter), and if you get light reflected from a nearby house or other building, that will further contaminate the light. Clouds or lack of clouds changes the light... all that makes it impossible for any kind of accurate calibration for bright-room viewing so all we can do is make the TV accurate without light in the room (while making it bright enough to be satisfying when there is light in the room) so when we increase luminance to compete with artificial or natural light in the room, at least the TV is outputting something fairly reasonable even if it isn't perfect because it's constantly being altered by ambient lighting conditions.
Yeah, OK, this TV is already calibrated and those statements do not answer my question or help me in any way. He is not aware of my ambient light environment - why talk about it??? Did I ask about it??? No, I asked one single specific question, which other posters help me with, but he went off talking about something I've known for a very long time now.