Unfortunately “optimal” speaker placement is very room/listener dependent. As long as you follow the general guidelines linked above the QS8s will do a fine job. The only real way to find the best placement in your room is to experiment with various placements. As a general rule I prefer surround speakers to be placed slightly behind the seating on the side wall. Placing them slightly back helps create a phantom rear image when you don't have rear speakers.
You will likely get better suggestions if you can post a picture/drawing of your room or at least give the dimensions and
