An easy basic step is to assemble a 90 degree arc.
Place the two ends at the edges of a sheet of plywood.
Measure from the center rail to the corner. Make sure both center rails are the same distance to the corner.
Double that number, that is the O-number.. The diameter of your circle.
If you reading is 36", it is 0-72.
If your reading is 27" it is 0-54.