As some of you know, my HT screen is a 4x8 foot (96x49") sheet of white formica, velcroed to the wall. Rather than trimming the sheet to 1.85:1 or 2.35:1 aspect ratio, I keep it at 2.00-ish:1 and zoom the projector lens so that 1.85 movies use the full height of the screen (89x49) while 2.35 movies use the full width of the screen (96x41).
To me, this seems ideal. Images are approximately the same size for both 1.85 and 2.35, allowing me to use a single seating position for both.
I currently have the projector mounted level with the bottom of the screen, using a lot of image shift so that the image is entirely above the projector centerline. The result is that the bottom of the image stays fixed when I zoom, and the top of a zoomed 2.35 image is level with the top of the screen with ~6" of black bar above the top of the screen.
I don't remember why I decided to keep the top of the screen as the common point between 2.35 and 1.85 movies, but after a while I realized that 1.85 movies seemed to be a bit "lower" than 2.35 movies even though the top of the image was always in the same place.
After a bit of research, I found that the rule of thumb seemed to be having the eyes level with a point 1/3 of the way up from the bottom of the image. Out came the calculator, and the result was that the centerline of the projector should be roughly 58" off the ground if I wanted the 1/3 point of both 1.85 and 2.35 images to be at my eye level (44" off the ground when seated).
That solved a nagging floorplan problem. I really wanted to move a sofabed from the living room up into the HT area but was having trouble with the current projector placement. Too low and the sofa blocked it -- too high and it would be out of reach for zooming. The new height seems to be ideal -- easy to reach for zooming, above most peoples' heads when sitting on the sofa, and (with the use of open shelving) at a height where there won't be much interference between ears and rear speakers.
I haven't set this up yet but it looks like it should work well. Just need to recruit some friends to help carry the sofabed up the stairs.
They say a good friend is one who will help you move a body, but that's nothing compared to a friend who will help you move a sofabed... even after the bed unfolded half way up the stairs last time
