See my comments: "The downside to this is that 20-60ish would be almost too fast, is that a bad thing? If so, should this only apply if Player hp > 60?"
I ended up scaling it by player HP, it works nicely if a mult of 3 is chosen: Assuming 3 and it rounds down this would mean no bonus until 67, *2 from 67-99, *3 from 100-133, *4 from 134-150.