Duane:
I do not think that you need the diametrical symbol for your drawing, Look in the ASME Y14.5M-1994 Standard, Para. 5.10.1 (c)page 141 for a description.
I think that all that is needed is a radius call out on the radius end's of the slot.
John Ruys
CMM Programer/Inspector
Teledyne MEC