Before tilt, my keystone was -5%. After tilt (arrived at by trial & error) and zero KS, the tilt pushed the bottom of the screen 2.5" away from the wall compared to the top.
Now this is purely anecdotal and possibly circumstantial/coincidental, but I wonder if the 5% KS was related to the aprx 2.5% tilt (relative to the diagonal of the screen, which is 104"). That would make it a 50% ratio. It would be an interesting theory to explore.
Is there a trig whiz in the house? My trig is way too old and wasn't all that good anyway. In any case, as others have said, the angle would have to be the same for the screen and PJ so as to maintain perpendicularity.
Edit: Upon thinking about it, the height of the screen is ~51", which makes the 2.5" of tilt ~ equal to 5% of the screen height. Makes more sense to me IMO.