In my opinion, 15% is way too high as people can put on VIT accessories and create a very large buffer time for themselves. Either keep it at 15% and don't allow healer to put on VIT accessories, or drop it down to 10% and keep it as it is. 15% is just way too high for the "average" player. Plus, this 15% changes impacts each healer differently. It makes a good Noctural AST/SCH pretty much immortal, pretty much making the threshold closer to 25%...

Have you ever tried to kill a SCH by yourself as a WAR when they have a crit adlo on them? Borderline impossible for a good 10 seconds. Not even a fell cleave will get through it. But I suppose that's the point - to make it easier for the more average healers while the more comfortable healers breeze through most of their matches with it.