I feel the break rate needs to be a function of the ATK of the creature hitting it, not of the number of hits it takes. The DR on this is huge when activated and it stops a large number of creatures. However if there are a bunch of small creatures, you may not be taking damage, but the shield is going to be much more likely to break. Making it a function of creature ATK makes sense thematically because those that hit harder should have an easier time breaking it, and the little guys will still have a chance, but not as easily as when break% is a function of # of attacks.