Hi Matt,
You can make your punch up to within about .003 or so of actual diameter. If you are getting it this close though, you want to be very careful to make sure everything is set up as good as you can, as it's getting mighty tight for the die/punch interface. Normally, .005 or so below actual size is a good number for gas checked slugs, and for plain-base stuff up to .010 under usually works fine. If you have a special need let me know and we can usually make up what you want.
Chris