The first thing you want to do is to get your seating in a position which isn't in a bad position in terms of room length. Start out in the range of 33-38% from the front wall to your ears. Once you find that position, play with speaker positioning and distances from side and front wall. Try not to have those dimensions the same in most cases.
There will be boundary response changes in the bass based on distance from baffle center to side walls and baffle front to wall behind the speakers. Shoot for the best balance of response and imaging.
The general treatment plan sounds OK. Don't forget the rear wall way in the back as your length modes will be VERY VERY deep.
If the speakers end up closer than about 2.5 ft from center to side wall, you'll want to consider a panel directly beside each speaker to tame boundary issues.
Depending on ceiling height you may want to consider diffusion at the ceiling reflection points. There may be other places where diffusion would be appropriate but it's hard to say depending on how live the room is, type of ceiling, type of flooring, etc.
For the corners, either build chunks ofr 6" or more thickness panels straddling the corners.
Bryan
