Here is the Center finder by Rob
Designed by 46rob
Another Center Finder
This center finder works the same way the machinists' tool works for finding the center of round stock. Print it on plastic sheet.
You just align the two 90 degree legs legs tangent to the edges of the circle, draw a line along the edge that bisects the angle. Rotate the tool a quarter turn and repeat the process. The intersection of the lines is your center.