Heh, all of us have a tinge of OCD or else we would not be here!
My very general suggestion would be to look at the PCM's for your rookie league (or your lowest minor league, assuming you have minor leagues; if you don't, start a test league with full minors and observe PCM's there). Then notice the drop-off of this lowest league from the one above it. Work out what the feeder league would be if the same drop-off occurred down to this level.
As with anything else of this nature, the best thing to do is to run quick test sims of several seasons or more to see if the results match your expectations. Tweak as necessary and test some more. Only when you are satisfied would you apply the final feeder PCM's to your ongoing game.