Markus explained why you _have_ to put the projector at the same level as the viewers: it's because the Da-Lite High Power screen is retro-reflective. In other words, it reflects the light back from where it came.
If you want to mount your PJ on the ceiling, you should try to sit/hang on the ceiling too... are you Batman or something http://www.avsforum.com/ubb/wink.gif
Anyway, yoy may have had good results so far with your ceiling mounted projector, because the High Power fabric is equivalent to a unity gain fabric when used out of its specified angle. In other words, if you keep your projector on the ceiling, you'll never leverage the "High Power" (is it interesting for a digital projector... I doubt it) but will still get reasonably good results. I presonally prefer a Da-Mat or any other true unity gain fabric, but the high power is OK.
Just for fun, I would recommend you to try a little experiment: just put your projector on a coffie table and try your screen the way it was intended to be used: you'll be amazed at the difference http://www.avsforum.com/ubb/smile.gif