For anyone may have not seen it, Cardas Audio,
http://www.cardas.com/content.php?area=insights&content_id=26&pagestring=Room+Setup shows a formula for setting up stereo or main speakers in a rectangular room.
It uses .276 and .447 as multipliers to the room's width. The resulting dimension from the .276 is the distance from the side walls to center of speakers.
The .447 gives the distance from the wall behind the speakers to the baffle, but that dimension most people would never use.
In my room (17.5 ft wide) .276 gives me 58" to the side walls, which is about what I already had, within an inch. The dimension, using the .447 would be 93" out from the wall, which I could use if the room was for stereo use only, but not for the HT layout.
Of coarse the narrower the room, the smaller the distances. But those resulting numbers, at least for the side dimension, is a good starting point, even if you have to cheat to be able to setup the screen
The closer you can get to the numbers, for your room, the better the soundstage.