Topper,
What I do is take a full length sized case, split the neck with a dremel, insert my bullet very shallow, chamber and remove carefully, this will be your length to the rifling. Do this a couple or four times, checking the measurement each time. You should come out with a consistent dimension for length to rifling.
Seat .020-.040 deeper, unless your throat is real deep. You need on a average the diameter of the bullet seated in the case, or runout gets critical.