Decide what SIZE circle you want.
Do the math. 360 degrees in a circle divided by the angle of cut = number of stones
If you leave the wide side full-size, 9.75 x number of stones = circumference in inches
inches / 12 circumference in feet
circumference / Pi (3.1415...) = diameter.
A 1 degree cut will get you a circle (technically a polygon) of 360 stones about 93 feet across.
5 . 72 . 18.6
10 . 36 . 9.3
If you make a cut on BOTH sides to make the stones symmetrical, the effective angle is doubled (10 degrees on both sides = 20 degrees = 18 stones = outside diameter is 4.65 feet)
ie /_\ .vs. |__\ for stone shape.
45 degree cuts on one side will get you an octagon (roughly) pinwheel with no place to put a fire in the middle, while both sides would get you a 9.75" square, still with no place to put a fire in the middle.
A hint from other media - try it with paper or cardboard before you commit to cutting stone. The circle formula gets increasingly incorrect as you use fewer and fewer stones which approximate a circle less well. The effect of one side or two side cutting also increases with fewer stones/greater angles.