I found this link when looking for a way to calculate the bolt circle diameter for the drive flange on my Honda GX610 engine.
You just enter the number of evenly spaced holes in the circle and the center to center distance between two adjacent holes. It works in metric and English measurements.
To determine the center to center distance accurately, use a dial caliper and measure the outside to outside distance between the holes, (the inside tips at the top of the calipers work good for this) then measure the circle diameter, and subtract it from your hole to hole measurement.
Hope you find it useful.