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.
Step 2: Find the Mid Point of the Chord
Now you know why it's easier to pick an easy length to start with.
Make sure it goes past where the center of the circle should be. You can go straight across if that's easier.
Step 3: Repeat Step 2 for Another Chord
This should be enough to find the center of the circle, but you can add more if needed.
Step 4: Use More Chords for Accuracy
Once you have the center marked you can use this information for other things, like drilling a hole in the middle, drawing concentric rings etc.
That's it, short and sweet.