Personally, since the character is paying -1 refresh for the ability to bypass some/all armor (and I would likely limit that to non-natural armor...although as I type that I could see someone coming up with justification for it's use against chitin as much as plate-type armor...so maybe I wouldn't limit it). I kind of like the idea of linking it to an Alertness or Lore (depending on whether its natural or magical) roll. Maybe the armor's value could be used as a "difficulty" to the roll and the shifts beating it could lower the armor by a like amount? The higher the value of the armor, the harder it is to find a weak spot, but on an exceptional result (from roll, base skill, or invoking/tagging) you could levy a greater penetration...