You can test the distance without having a projector. Go to a store where they have a good projector and screen set-up; move back and forth. Establish where you feel comfortable (using multiple types of video material)then measure your distance to their screen and do the corollation with your installation (projected seating distance and screen size).

I helped a friend installed his Panasonic 4000U a few weeks back; it is a superb machine. I own a 2000U and the 4000U is much better.

jc