does magic resistance add up?

[asmutny]

a simple question about the game mechanics:
my black and ice dragons have both magic res at 80%, blackies also have fire res at same level. I figured equipping 3 crystal balls to the hero (each giving +5% magic res) + 7% all dmg res from Guardian Angel medal should give to the dragons full 100% magic resistance... but it didn't and Armageddon spell still deals more or less the same amount of damage to my dragons. why? did I get it wrong or is it somehow bugged?