So I just used the probe and by hand found the center of one of the button holes in my case.
Was then able to chamfer the edges.
Not bad, not great. Was a hair out on the Y. X was perfect.

I just added 0.025mm to the Y and it is near perfect.
So Zeeflyboy your method works... :p

The real test would be machining around my part top and bottom now with out much, if any step in the surface finish once flipped and had the same operation done.

I guess the best way to do the flip is to leave 0.0something on the bottom of my stock and then flip, then remove that last 0.0something and chamfer...?