Programming MSD - when should timing be all in?

You have to think backwards on MSD. You set your total timing and lock the distributor. No centrifugal or mechanical advance. You then retard the timing through the MSD box. It only lets you take away 25 degrees, so if my total timing on race gas is 38 degrees, I can start at 13 degrees, ramp it up at any rpm of my choice for idle, then make any curve I want. I'm looking for thoughts or for knowledgeable ignition guys who might be able to tell me when they think I should be all in.