According to the FCF on your sketch, you can not assume that datum C is the primary. The order in which the datums are called in the FCS determines which is the primary. I am unfamiliar with using both A and B in the same section of the FCS, however I do know that C can not be the primary because it comes last.

If I were inspecting this part, I would want to set up the coordinate system and Geoff explained, but if you do, you will not be following the print.

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