of course this is all room dependent, but I go with an equilateral triangle (that would be, in your case 7 feet speaker to speaker and listener to speaker-i) to start, and then move my chair back a little at a time until I have perfection. In my room, now, I had to move my chair back a foot and a hair. I like a deep soundstage. Can't move back too far, or there it goes. sounds good to me
P.S. and that's a big P.S. remember the distance from the speakers to the back and sides. Major factors as well. You don't want your listening chair close to the wall behind you, either. I'm sure you realize this, but just in case....
