Slow, welcome to Axiom. As has been pointed out, the exact height isn't crucial, particularly with wide dispersion surrounds like the QSs. The most typical suggestion is 2-3' above ear level.
Something to keep in mind is that the surrounds in a 5.1 setup are still side surrounds, even when mounted on the back wall. This leads to a suggestion to spread the surrounds out more than shown in your diagram, so as to give more of a "side" effect than a back effect. Moving them both out at least another 2'(if the window in the right corner allows)should be considered.
Another suggestion is to place the sub directly in the right front corner, for stronger room reinforcement. If not directly in the right corner, moved out about 5'(1/4th of the room width)from the side wall.
