The right screen gain depends on quite a few things. Obviously, the projector is one of the big items to consider, but the room configuration and lighting conditions are other factors. In order to help, simply answer a few questions:
1. What is the ANSI lumen output of the proposed projector?
2. (a) Do you have control of the ambient light in the room?
(b) If not does ambient light fall directly on the proposed screen location?
3. What is the seating configuration?
(also) How far back and wide is the first row of seats?
4. How large a screen are you contemplating?
With those bits of information, one can make a better informed decision when it comes to choosing a screen gain.
Hope this helps.