basically its the multiplier of your base damage that determines the additional style damage you get.
In other words, if a style has a growth rate of .7 and you hit someone for a base damage of 100, you will see in your combat log:
Wow, I don't even remember exactly how its written in the combat log.... ??
You perform some style perfectly (+70)
You hit for 170 damage. Is that how it works? lol
When you swing an enemy at a person it always varies in damage quite a bit, that's because the base damage always varies, however growth rate supposedly remains constant for whichever style you use. (of course I mean constant when you have the same spec/stats/equipment). This is easily testable however.
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