All manufactueres recomend a certain projector offset according to the height of the screen. THis dimmetion does not effect focus however, it effects keystone. This is the trapaziod effect you get from tilting the projector too much up or down. there are 3 ways to address this problem. ONE: Move the projector to the right location... On some projectors, this is your only option. TWO: Lens shift. Many projectors have the ability to tilt the lens to straighten out the inage. THe THIRS should never be used. that is Digital Keyston Correction. This is a circuit built in to many newer DLP projectors that takes the native output, Scales it down, and then digitally shrinks and skews the image to produce a trued 16:9 (or 4:3) image.

As far as focus is converned, no worrys, the lens is designed to keep the image uniform, on some machine, you may notice a small difference on the edges, but most things you watch keep your focus on the center of the screen so it rarely becomes an issue.