Back surround speaker placement ?

[da5176]

Hey guys, I need some suggestions for back surround sound speaker placement. I have a 7.1 speaker system. The room is 19.5' X 12.5' X 7.5'. The side surrounds are going about 3.5' from the back wall facing toward one another. They will be about 3.5' behind the main listening position. Any more forward to the sides of the main spot can't be done due to a 4' wide window. The question is this, the back wall is wide open for back surround speakers. The distance is 7' from the back wall to the main spot. How far apart should the back surround speakers be? Should they be at the same height as the side surrounds? I'm re-arranging furniture, flipping from one end to the other. Wife wanted a change, and think I can get better surround speaker locations than before. We have more room this way so I need to make things work. Any help would be appreciated, Thanks

 

[da5176]

Pictures

Here's some pics to better see what I'm asking about. I was thinking, what if I mounted the sides from the ceiling to bring them more to the side of the couch? If I did that the right side would be somewhat in front of the window, the other above the door. How would that effect sound? Or should I stick with the first idea of to the right of the window, left of the door. The speakers are Polk Monitor 30s, front firing, weighing 9 lbs. Thanks guys!