It's very simple, get one of these
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If your reading 30ftC at screen, facing projector and say you want 15ftL reflected of screen get a 0.5gain screen
ftL = FC x screen gain
I would recommend 14-16ftL, problem is if you have never seen that for any prolonged period of time you won't know if you like it.
If you like commercial cinemas, that's what their supposed to use when their not trying to save money, further turng brightness down. A good example is imax.
Lowering peak white to 15ftL doesn't give dimmer whites, it gives perceived brighter whites because of improved contrast.
Your eyes have a contrast range say 1,000,000:1 and as the environment gets darker or brighter your pupil closes or opens extending the levels of brightness that can be perceived.
What 14ftL does is keep your pupil relatively closed, but you can still percieve that 1,000,00 contrast brightest whites and darkest blacks
Also your pupil constantly opening and closing can cause eye strain.
If you go much lower than 12ftL then yes the picture will appear dim and you will lose perceived contrast.