Kind of a general CMM question. I'm running an old Numerex 24-28-18, PH9/TP2, CMM-M 12.0.1.8123 . I'm hoping to see if anyone has any advice for getting more accurate diameter measurements. 

After qualifying on the calibration sphere, and writing a small DCC program (8-point circle) to measure a 2.2500 XX ring standard, my machine still tells me the ring is .0004-.0005" circularity. It gets the size within .0001" (least squares), but undersize .0004-.0005" (max inscribed). 

A XX ring is obviously much rounder than this. Do you have any suggestion on how I might eke out more accuracy out of this machine, or do you think I'm at the limits of this machine? I have some parts coming soon that are asking for a .0002" circularity, so I'm hoping there's something I can do.