A pretty sharp feller at AVS answered this very question for me once. He gave me some simple math to use which is…..
If you want to follow THX recommendations for viewing angle, divide your room length by 2.17. That will be your max screen width for the room.
Then divide that width by the screen aspect ratio you wish to use. 2.35, 1.78 or 1.33. That will give you your screen height.
Then to find your maximum seating distance, multiply your screen width by 1.54 or, your screen height by 3.68.
One thing to consider is screen height off the floor. If it’s too high, you get neck strain. Too low and you can’t see. 32” or so seams to be about right which runs into problems when you don’t have much room to either side of the screen for tower speaker placement (like my situation).
My theater room is 12 X 16. I have enough room for a couple couches, but definitely not enough for two rows. I don’t think 18 foot would either, but possibly if you can put one row right up to the back wall.
