SD, when similar questions have arisen in the past, my view has been that the back surrounds(yes, even with QSs two should be better and allow for a "stereo" back effect on some passages)should be at least 4' behind the listening position to give room for a rear sound field to form. Because of the exceptionally wide dispersion of the QSs, even 3-4' back would give a reasonable effect, but twice that distance should be better if available.

As Dan pointed out, you've given distances behind the side surrounds, which may not be the same as distances behind the listening position if the side surrounds are farther back than the listening position. Be that as it may, placing the QS8 about 9' back on the rear wall and about 3' above ear level should give a deeper back surround effect(assuming that the path is unobstructed)than would result from placing it on that 8" partition about 8' up and 3' back.
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Enjoy the music, not the equipment.