Ok, I have to ask this about ceiling height, lens shift and keystone. Lets use PT-AX200U as the unit for example as I am strongly considering this unit.

My ceiling height is 7'6".

My screen is most likely going to be 100" (49"x87")
I plan on having the top of the screen aroudn 12" from top of ceiling.

The PT-AX200u will be ceiling mounted (centered horizontally with the screen) as close to the ceiling as possible in a soffit or near flush mounted.

The specs say that I can do a vertical off-center shift of up to 63%.

I obviously will need to do a downward lens shift within the allowable shift???

By lens shift, does that also mean keystone?
If they are seperate, then will lens shift degrade the picture quality?

I have that PJ and am in the exact same situation. Lens shift is not the same as keystone correction. You should be well within the shift spec. The PJ will not be flush to the cieling in my experience. (limited as it is). My lens is about 6 inches from the ceiling. That would make it only 6 inches above your screen. I have mine mounted level and use the lense shift to move the image down without any use of keystone correction. I will say this, mount your PJ as close to the center as possible. Any horizontal shift cuts down on the amount of vertical shift available.

I have the same size ceilings in my basement but i have the hd70. i'm going to get a ceiling mount but i'm going to install a 2x4 between my rafters (going to cut the sheetrock to expose the rafters) and i'm going to measure it so the projector is basically on the ceiling w/o much wasted space. I think i can get a 96" screen w/ 13' throw. hope it works....ordering mount today.