given your room width of 11', 8' of separation between the towers should be good. But my suggestion was to help you have enough clearance between the towers & the side walls.
One more suggestion. Since your room is under construction, see if you can plan & keep your audio rack somewhere in the side - i.e, not directly in front of you & in-between the speakers but somewhere along the 17' long wall. This way, there will be no furniture/ clutter in-between the towers. I believe this makes a big difference. Most of the listening rooms that I have been to, had their audio racks in the side. You will have to plan your wiring etc based on this if you want to implement it.
