At viewing position (usually seated), you want the eyes to be dead center on the screen if possible. If not, then you'll want to see if you can get away with angling the unit down using a bracket that can be manipulated.
There's a reason for the average height of rear-projection TVs. To get a rough approximation, you can simply go to Best Buy or Circuit City and take some measurements to see where the center lines average.
Hope that helps!