You are right, this is discussed on page 3 of the screen simulation thread:
|With an optimized radius circular screen you have to balance high gain with brightness uniformity. You cannot go too high with gain as the vertical brightness uniformity will suffer and you cannot go too low because of cross reflections.
I believe you probably should not use a gain lower than 2.0 to avoid cross reflection problems. The 2.0 gain Stewart Ultramatte 200 will give you a minimum cross reflection rejection ratio (CRR) of around 25 for a 3.5 m wide, 4.4 m radius circular screen, 5 m viewing distance and horizontally centered viewer. This means that a bright area to the very far left/right of the screen (worst case situation) will produce a "ghost" with some 4% relative illumination on the other side of the screen. Under the same circumstances, the 3.2 gain Stewart Silver 3D would give you a whopping minimum CRR of >700.
What the above means is that you will have no practical need for a higher gain than around 3 when it comes to avoiding cross reflections. This is true for spherical screens as well as the circular one discussed above even if the actual numbers may differ slightly.