Water Powder

At how much additional water damage does the water powder special on armour (concentration) cap out?

For Endurance, it seems to be 40% per armour piece.
But what about water damage for conc.?

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Also, do I see it correctly that concentration always overwrites with the most recent spell cast?
If so, that pretty lame, considering you only get a one-time damage buff that is not even that high (1k at like 18 mana cost) for only one spin, which instantly overwrites for the next ones.