Thanks to everyone for the variety of responses.

I went to my local Home Depot and they did not have the centering ruler that Brian Shaw was referring to.
I went to an upscale area several miles away and visited their Home Depot and they did have the centering ruler and I bought it.

I have found this to be true for several different retail companies. The stores in my area, I live in a "Hub Zone" area , do not carry the selection or quality I find in the upscale locations.
So I was not surprised to find the ruler that was mentioned in the upscale location.

Thanks again for all the suggestions.

Gary