Actually, given that the only class that should get Category A Class Defense is the monk(who does have innate AC enhancers), you can just give them B and let their innates cover the rest.
For the matter, unlatch it from BAB(though its a decent rule of thumb), and make them class specific:
Fighters, Barbarians, Rogues, Monks, Paladins -> 3/4 class level, these are for the front liners and naturally evasive classes.
Clerics, Druids, Duskblades -> 1/2 class level, either for those who need not engage in close combat or who habitually rely on magic for defense while doing so.
Wizards, Sorcerors, Archivists ->1/4 class level, these don't belong in close combat.