The answer to your question is not as straight forward as you imagine. However, I do have a workaround for you, you can position temporary planes tangent to hole faces which will let you select a reference dimension. Be careful, I tried to do this with axis and they are not so nice when it comes to behaviour.
Please look at my images I attached.
Good luck, hope this helps,
Thanks for the quick answer, I'll experiment with this approach a bit -- looks useful for some applications.
I know the feeling, TGIF :]
I completely forgot about the arc/circle measurement drop-down box, thanks for the fast reply.