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Hi Tony, The octagon can be divided into three parts: a central rectangle and two trapezoids:
The trapezoids will each be 12/sqrt(2) feet wide and the rectangle will be 12 feet wide, for a total of 12(1 + sqrt(2)) feet [you can see how to scale this for other panel sizes.] This works out as almost exactly 29 feet, side to side. If you measure from corner to corner, it will be slightly larger (by a factor of sec(22.5 degrees)) and the diameter will be about 31 feet 4 inches. RD  


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