Is there any real way to avoid Overextention when playing a major power like Castille with colonial ambitions in DW?
From the Wikki it appears that you really can't have high number of provinces and a high centralization, which means you're stuck with the negative modifiers, right?
If you have 85%+ of cores, then you can have any combination of administrative monarch rating and centralization as you want.
If 75% cores or less --> must have ADM4 4+
If 66% cores or less -->ADM 6+
If you have a ratio of less than 50%, then you gotta worry about being decentralized, but I honestly never do, because having such an overexpanded low-core empire is usually transient.