I'm a newb here so if this idea sucks or has been done, I appoligize.
Can someone with good control of light try this and report the results.
Can you increase the amount of light until you think that the picture becomes too washed out to be watchable. Then using a light meter or a typical SLR camera, turn off the projector and measure the amount of light in the room. Sit at your normal position and aim the camera at the screen.
Use the standard 50 mm lens if possible and record the numbers for the f-stop, ISO, etc.
Please describe your room setup in as much detail as practical including projector. The most usefull for me, would be data from rooms with blinds, however if enough data is collected from several room setups we might come to some sort of consensus for each type of room.