Position to a single cylindrical datum

[[Ke...]]

I'm measuring a part that is similar to this sketch...
where the Base Alignment is set to AIBIC ; A is spatial, X zero & Z zero; B is Y zero, and C controls rotation.

The feature being evaluated for position is small, and difficult to access, so I am using 4 symmetry points, along the feature.

For the Position callout:
Using the A datum only (whether in 1 box or all 3), the position is (e.g. 0.350mm); whch is similar to evaluationg it to the Base Alignment (A & C are 3D lines in the Base Alignment, while A is measured as a cylinder for evaluation purposes)

As reference the parallelism between the feature and Datum A is 0.0143

However, If I use a secondary alignment, and use A for Spatial & XZ origins, and control the rotation with an Average Feature, derived from the 4 symmetry points... my result is 0.0298 (very close to 2 x the parallelism result.

Is the CMM adapting the rotation from the Base Alignment, instead of allowing A to rotate freely?
If so, using the average feature to clock the rotation and acceptable modification?

FYI: I have Curve, but not Freeform (at this time...)

[[SH...]]

If I have not mistaken, designer is looking for symmetry of your feature wrt the datum A. I think no need of clocking.

[[Ro...]]

Hi Keith,
Are you best fitting the DOF not constrained by datum A in the position? If A is a cylinder it should be rotation around A and translation along it's axis.

[[Ke...]]

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Thaaat's the ticket...

Thank you sir