Interesting trick...guess I forgot my geometry.

I always worry about the side walls being exactly 90 degrees to the screen wall. If one were to measure from the same side wall but the angle of side wall to screen wall was off 1 degree or the builder didn't plane his studs after the framer used twisted pieces of crap you could be off. I mount the screen... measure to the center of it. Then I break out my square (36") and make a few tiny marks along the side that juts out. I then put my little 39.00 Home Depot laser unit on the ceiling and project it along my square line. I get the lens smack dab in the middle of the red and rarely have more than a tniy adjustment. Always open to better methods though.

Keystone adjustments are for teachers showing slides or corporate suits with Power Point Presentations.