I've been measuring with CMM's for over twelve years now. The only way to measure the radius would be to set the center location of the radius with a gage point, get into the polar mode and take points at the known angle and radius.

Several things keep you from using the circle or hole commands first the math involved in the least squares that is used to calculate a hole location will give you data however, if you are not measuring more than 180 degrees of the circle the data will be wrong.

Our company does not measure radius with the CMM and identifies only the intesections of the surfaces. Use a raduis gage to measure the actual rad. or take the other persons idea with the mold and use the shadow graph.