Ross;
All of the GD&T classes that we have taken the instructors have said that if we get a part with a symmetry call out we should have the engineer explain way he/she needs it because of how much time it takes to inspect. What ASME Y14.5M 1994 says is " Within the limits of size and regardless of feature size, all median points of opposed elements of the slot must lie between two parallel planes " " the two planes being equally disposed about datum A. The specified tolerance and the datum reference can only apply on an RFS basis " FIG. 5-61 on page 155 of ASME 14.5 M 1994. This is for a slot but I think it still apples in your case, I think I would see if a POSITION or PERPENDICULARITY call out would not work out better for you.

John D. Ruys
CMM Programer/Inspector
Teledyne MEC