Can it be assumed that your main listening position is in line with your side surrounds, so 3 and 9 feet back are also in respect to the listening position?

If so, based on my mental picture of what you describe, I am thinking the 3 foot position. Might the arch interfere/block the back sound field if the 9 foot position is used?
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Dan
On-Wall M5HP LCR, QS8 & EP500 in 7.1