Crusher with a polearm

So this is where the extra mastery comes in for DrKs? I had always been confused as to why, even with 80% mastery, our min was barely less than half our max, while heros get more than half of their max in their min with 60%.

I'm almost embarrassed to say that I had been puzzling for so long as to why I wasn't hitting 11k crushers -_- There goes my knowledge of the game.

So if I wanted to calculate average spear crusher damage, what's the real formula I would use? (I'm confused as to where the .72 comes from in Dusk's calculation up there)