Thanks for pointing out that the High Power is retro reflective. I suspected as much, but I was not completely up to date on the Da-Lite products.
A retro reflective screen reflects light back to the source. This means that your viewing position has to be close to the projector. Unless you like hanging from the ceiling, this is not a good choice for your setup http://www.avsforum.com/ubb/smile.gif
When the angle from the viewing position to the screen normal is larger than some 20-30 degrees, the gain will drop below unity for a high gain screen with the projector placed directly in front of the screen center, just as Gerald noted. Placing the projector below or above this position makes things slightly more complicated in the vertical plane.
I would much prefer if the term gain could be substituted for directivity, which is a much better technical description of what is really going on. The term gain really seems to make many people assume that they get more light output from certain screens than from others. This is, of course, impossible for a passive device such as a screen.
What is actually happening is that a wide reflective cone is concentrated to a very narrow one with higher 'brightness'. Remember that total light output has to be the same (neglecting reflective losses) regardless of screen material. In other words, the area under the gain vs angle curve has to remain constant. If it does not (as I have seen in some cases) - well, either someone has formed the basis for a perpetuum mobile or the same someone is full of B.S.