The spell Shield is supposed to (if I read it right) function as a turn/level mage armor when you have GSF:Abjuration. However unlike Mage Armor, when this spell is cast it factors in any pre-existing armor bonus and removes it from the total granted.
Warder robes for instance, offer +1 ac. When Mage armor is casted on it they gain +4 ac, yet when GSF:Abjur Shield is casted it gains +3
Hopefully fixed, let me know if it's good or not.