Be careful with probes to find circle center. I have been doing this for a while with a probe from IMserv. The other day I decided to check the accuracy of the center and found quite an error.
If you run the probe once and find center, then rotate the probe 90 degrees and do it again you may have several thou error.
I talked to Fred Smith of IMserv and he said that was expected of a probe- they are not made to be concentric and you need to first probe a known reference and set compensating values for the error.
I bought a Wildhorse Innovations probe. It has a much stronger spring, and is designed to let you make an adjustment for centering. I now have mine so Im within 1 thou of true center.