Can you offset the probe repeatably? It would seem that the ball position is not as important as the whole probe's touch position (where it indicates an edge, even if the ball touched 0.004 earlier). That is what you really want to center, so try offsetting the probe until it touches (electrically) as uniformly as possible.
Also, the effective diameter of the touch position may differ from the physical ball diameter (and may even be an ellipse). I would think that you want to use the effective radius, not the physical radius.
You can also leave the probe alone and do the offsets in your head, then subtract them every time you touch off of something. Just remember the offset when approaching from each cardinal direction.
I don't have a probe, but I thought that Mach allows you to set up those kind of offsets.
- Just Gary