If you get the center of lens and center of screen perfect, then the projector will be square to the screen when the image is centered in the screen. ;)

I tried the laser, and went back to old school as mentioned above. String (or CAT5 lol) and a pencil. Make 2 bisecting arcs and center the lens on the screen. Seems caveman-ish I know, but it works everytime.

Its also nice to use projectors with horizontal and/or vertical lens shift. ;) Fudge factor is always nice.

One thing I learned at Runco Academy was NEVER USE KEYSTONE ADJUSTMENTS! Screws the resolution and geometry.

You mount the screen where you want it, and you put the projector where it needs to go height-wise (look in the manual for offsets). Mount the projector so it is level and center the lens on the center of screen. Calibrate. Done.