This Axiom link might help. If you're seating is right against the back wall I might be tempted to put them on the back wall (at the correct height as Jay said) just to make sure they are a bit behind you. However the QS8's are very forgiving on placement so anything reasonable placed should sound good with calibration.

Dan
On-Wall M5HP LCR, QS8 & EP500 in 7.1