I've said it elsewhere before but the DC by Level and Difficulty Alteration charts will handle most questions a GM will ever have about the system.

even running an AP, those two tables are so consistently useful, both in and out of sessions. and even going outside of them, I'm consistently blown away by how many useful tools there are! I needed to set a price for an item and searched on a whim to see if anyone had made a guide on doing that,