I read through the guide on Mana Mastery and found that the formula posted there doesn’t give me the mastery my hero page shows.

My mastery is 153 for blue for example. The guide states the formula is Surge Chance = Mastery / (100+Mastery).

So . . . 153 / (100+153) = 153 / 253 = 0.605

Now rounding down I get 60% but my hero page shows 51%. Has the formula been changed in some previous update?