If you want to powergame the heck out of your regent, and gain 100% regency from every kind of holding, then try out this build.

Wiz 3/Clr 3/Mystic Theurge(DMG) 4
requires 18 INT, or 16 INT+Human

Spellcraft 4 Ranks
K. Arcana 10 Ranks
Lead 10 ranks
Administrate 10 ranks
Diplomacy 13 ranks
Craft(any) 13 ranks
Profession(sage) 13 ranks
Sense Motive 1 rank
K. religion 6 ranks

All this assumes that your DM will rule that all of the additional skills in the BRCS are cross-class skills for the Mystic Theurge PrC. By 12th level, you will be right back at max ranks in spellcraft.