Our priests from Spain have been testing and the conclusion is that Divine Aegis absorbtion depends of the stamina of the priest, concretely 40% of the caster's health, and not from spell power or intellect. Mastery also affects Divine Aegis about 2.1%/mastery point.
It's a curious not really useful consideration since enchanting or geming stamina is ridiculously inefficient, but just to inform you.
This is the thread if you want to read it : http://eu.battle.net/wow/es/forum/topic/2868712028