What I mean is getting into grooves to measure the top and bottom (width)  or max diameter when the groove is not significantly wider than the probe you are trying to use....

I had a part today that had a 4.25" Ø ID groove, that looked like I could measure with a 2mm Ø star probe, it is tight width wise  0.082" But I thought I could set the approach and retract way down and get CMM-M to measure.  It would not....

I could measure the groove max OD, because it is a radial approach and retract.

What is the minimum ratio of groove width to probe diameter that I can expect to have a successful path creation ?