Screw that mastery and get colo water stone. It can go up to 200 (or 180 if you want to leave it at +8) and contains the highest water intensity of all stones in the game.
It has the highest water intensity of all stones of the same level, for that matter.
The 4th/final mastery is ultimately useless. 20 X intensity is pretty nice, but you'd get more damage if you were to just level up your favorite skills instead. If you have extra points, work on your buffs. If your buffs are all done, work on your favorite attacks, if those are done, work on your 5th tier, etc, etc, etc.