User blog comment:Khaladar/The Bonecrusher/@comment-5009341-20120608183144/@comment-5159141-20120609125316

We must not forget that everything has a counter. No matter how much damage this does, it only makes the Thornmail more punishing. I even think that buying a thornmail would be such a good counter to this that somebody would rather sell it and buy a BT than keep it and get hurt A LOT.

Regarding your question, percentage values tend to stack multiplicatively, and they do in this case as well.