"Screens with gain may perform better than that observed if the projector is mounted closer to the observers eye level."
The preceding quote taken from your otherwise excellent report is not 100% accurate. It is absolutely accurate for retroreflective screens, such as the Da-Lite High Power. It is not quite right for angular reflective screens, such as most of the other screens in your report. Angular reflective screens exhibit the greatest gain at screen center when the angle between the projector's line of sight above screen center and a line perpendicular to screen center is equal to the angle between the viewer's line of sight below screen center and a line perpendicular to screen center. In other words, if the viewer's line of sight is 10 degrees below screen center, a projector position 10 degrees above screen center will produce the greatest gain at screen center. Anything closer to (or further from) the viewer's eye level will produce less gain.
Your statement is true for both types of screens only when the viewer's eyes are at screen center height (which may?? be true for your theater). In the general case, in which the viewer's eyes may not be at screen center height, your statement is not true for angular reflective screens.
You don't have to take my word for this. If you move your measurement device below any angular reflective screen at an angle below screen center equal to the angle of your projector above screen center, you will find that to be the point of maximum gain for a point at screen center. (Angle of reflection equals angle of incidence.)