Walter, the 6' up and 10' separation were just ball-park numbers. You should leave at least 6" of space below the ceiling for the top speaker on the QSs to work with. They should also be above ear level, and 3' is a commonly suggested figure. The reason for having a wide separation is that even if the speakers are mounted on the back wall, they're still side surrounds and should be separated enough to give more of a side effect than a back effect. You should try to balance those two requirements as best as possible within the limits of your room arrangement.
