I have had this same situation many times. I have found that the symmetry points method is somewhat unreliable due to you inability to take the opposing points at the exact same location. What I do do is measure a line on each side of the slot and intersect those lines with Datum B (either the ID or OD) I then ask for a symmetry point between those 2 intersections. I find this to be very accurate. Just one method of many I am sure.

Bryan Sorensen
Quality Manager
Tosoh Quartz Inc, Austin
512-929-8257