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Projector

Screen

For a 1080p HD projector, a good rule of thumb for viewing distance is 1-1/2 times the screen diagonal. So, for 10ft viewing distance, diagonal should be 6.66 to 10 ft.

Gain

The amount of light that a projector reflects back at viewers.

One down side to higher-gain screens is that they can suffer from “hot-spotting,” where part of the screen looks noticeably brighter. This is because they reflect the light back in a narrower, more focused band. The picture will look best to viewers who sit directly in front of the screen rather than off to the sides.
If you have a wide seating area, you might want to consider a screen with standard gain as it will reflect the light evenly in all directions. This is also called neutral gain, or it may be listed numerically as a gain of 1.0.
Negative gain screens help with ambient light issues - and are oft used with high-lumen projectors.