Bradz,
You asked about viewing distance with a 50" diagonal 16 X 9 screen. The maximum SMPTE viewing distance to obtain a 30° viewing angle is 6.8 feet; the maximum THX viewing distance to obtain a 26° viewing angle is 7.9 feet. The recommended THX viewing distance is 5.6 feet. These distances are aimed at providing an immersive theater experience for movies.
Front projection matches these viewing angle requirements, because the diagonal can easily be 100" or more, but front projection requires a darkend room, a darker environment than you might wish for casual TV viewing. Then there's the bulb life issue.
In any case, I don't think you want to sit 12' from a 50" image. I've read advice of viewing distances ranging from 1.5 to 2.5 diagonal, but 1.5 to 2 seems more reasonable -- assuming you want some immersion, not just an image you can see. (The maximum vieiwng distance for a 50" screen is 18 feet)!
db