Nope you have an ellipse...
The radius for the 9'6 points is 6'8 5/8 which leaves you 23 5/8" at the center of the arc.
A sq + B sq = C sq the hypontenus theory.
4'9 being A & B leaving 6'8 5/8 for C
6'8 5/8 minus 4'9 at the center leaves 23 5/8" not 17" I think anyway... I could be wrong?
Try putting 2 nails 9'6 apart and let a string droop down to 17" that may be your best way to get your answer.
By the way the tramel Mark speaks of can be made with a 1x4 with a nail for the compass point and another nail to mark out the arc. I do that for arc top windows & even laying out high rise entrance circles... works great for the trim to put two nail on the making end. Gives you both diameters.
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