I hope this helps. I am into CIH (Constant Image Height) and created this really easy to use diagram in Sketch Up. This is based on that chart that Gary posted on page 1 and is how I work out screen sized and seating distances.
The two images are the same, I have just scaled the 2nd up to 2x the size keeping the girl at 1:1 for a scale reference.
Both take the room length and divide that by 4 to find an ideal screen height. This was actually the recommended method back by CEDIA in the 1.33:1 days and works perfectly well today.
The screen height is then used for both finding the width (by multipling by what AR you desire) and the seating distances. In the chart posted by Gary, there is a minimum of 2x the image height recommendation by SMPTE. This can also be found on the Disney WOW Blu-ray and in real life experience (from someone that actually sits at this distance in their own cinema), it is very immersive.
There has been discussion about the THX 36 degree recommendation as well. Based on the chart posted by Gary, 36 degrees is about 3.68x the image height (for CinemaScope 2.39:1) and I have used that to find the farthest distance from the screen. Note it leaves a gap between the back seats and the back wall.
To keep it really simple, divide your room by 4 to find the image height. You sit NO closer than 2x and NO farther than 3.68x and you can feel free to sit ANYWHERE in between you want.
I have used the mid point between 2x and 3.68x to find the centre of the surround field. I have used equal spacing of 60 degrees between each surround. Even if seated behind the side LS/RS, you will still get good envelopment from a 7.1 system because you are still in the sound field.Edited by CAVX - 3/4/13 at 2:22pm