I would bet a Dime against a Dollar the Shelf / Box...as it were being mounted to the Rear Wall...will be far enough away...but probably not over 20'.
That 89" Ceiling height, after a shelf is constructed that has at least 10" clearance between the shelf and the Ceiling, will leave the bottom edge right at 6'.5", and theLen's Center point at 6'-9" (...which is 8" below the Ceiling...) so in actuality, any shift will be more directed to a higher point than lower. So there is no reason really to warrant much discussion on that point. Such a revelation earlier on might have reduced the superfluous content to a considerable degree. Again the point is made...when asking for advice, provide all the known information you can. And if such has not been provided, before dispensing advice / opinions, ask for such. I tried to elicit the Ceiling Height figure early on....but it didn't come until after 9 posts later so...*
At least the OP is solidly in the driver's seat as far as having sufficient brightness (lumen output) with a JVC coming on tap. Even at 20'-4" he will get 49 fl off a 1.0 Gain 100" diagonal surface.
All he needs to focus on now is building that Shelf solidly enough (...and deep enough...) to support a 34 lb load at a retained "level", and get himself the best possible deal going on his JVC.
, Here are some very pertinent Qs.
- Do you have any access to the Ceiling above (...sounds like a Basement with that low Ceiling...)
- How cosmetically pleasing do you want the structure to look? Minimalist?
- Please relate your actual Room's depth from rear Wall to Screen.
- Where will your seating be in relation to the PJ's location? (...has much to do about any need to "Hush" the Projector...)
Starting out, I would vote for a minimalist approach, a shelf with no Sides, and that has a Base that can be removed and replaced with one that may be either Larger...or even Smaller, as well might be the case since the JVC is so beastly large its highly improbable that your eventual upgrade will require even more of a footprint area.
A Shelf 19" Wide and 20" Deep ...or 20" x 20" (...with the necessary 110v / HDMI cordage exiting the Ceiling at the rear of the PJ...) would be more than sufficient for now and the future. If I was building it, I would use a 1/2" thick Top Plate of Cabinet Grade Plywood set in Flush with the Ceiling's drywall, solidly anchored to the Ceiling Joists or a combination of such and extended Deadwood. The Base would be 3/4: Cabinent Grade Ply. Using 6 -1" diameter Dowels, 3 down each side, Each location through the Top & Bottom Plates, as well as into each end of the Dowels are Pre drilled and attached with 2.5" Coarse Threaded Screws would rigidly support the suspended shelf assembly, allowing for the Projector to be "Front Loaded" and slid back until the Face/Lens is flush with the Leading edge .
Here is a basic Diagram: