Lots of you guys may have come up with the centering tool idea on your own, but the idea has been around for centuries. It's still a good one. Ideas similar to Eric's Gizmo have been around at least since the 1930s, but I think the earlier ideas were unnecessarily more complicate--some used a dial indicator in place of the pencil, for instance.
I think Eric's approach was unprecedented in its simplicity and utility--different from any earlier version I have seen and better.
Jim Davis