You might want to use this technique to know where to drill the hole in the middle or draw concentric circles on the surface.
I can't take any credit for this as I probably learnt it at school, many eons ago. I just thought I'd add it because I saw lots of methods for drawing circles, using bits of string, wooden battens etc. but nothing for reverse engineering the problem.
If it's not correct then I can only blame my very poor memory.
Step 1: Draw a chord across the circle
If you can also make the chord a nice easy length i.e. 10, 20, 24, etc this might make life easier in the next step.