My post is based on the following post, that was confirmed officially.
Else the whole mitigation % would be meaningless. The formula even is in the ingame tooltips since the last patch. Why do this when it is not calculated this way?
May be that there are additional mechanics that calculate how much damage an enemy deals depending on your level or something else. I don’t know.
I hope I am not telling nonsense. I only repeat what I’ve discovered here on the forums so far. But I cannot believe that protection acts like a shield so 400 protection blocks 400 health and you only take the difference in damage above 400. This would make this discussion meaningless.