Cutting stone at an angle in half to make a trapezoid for ...
Assuming my calculations are correct, I'd cut the 12" stones at about 6.4" / 5.6", flip the stone where I made the cut, and create a trapezoid with 12.8" on one side and 11.2" on the other. This trapezoid would then have an out and inner circumference to match the size of my 11' circle, and the 4" width blocks.