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With Frailty it's similar: TTK is 85s base So the DPS is 100/91.43 = 109.375%, that is 9.4% bonus DPS you get from Ruthless Let's say normally your TTK is 100s - and now we add Ruthless: TTK is always inversely proportional to your DPS so if you figure out the new TTK, it's easy to get DPS bonus from there. So the best way to calculate this bonus is by thinking in terms of Time To Kill (TTK).
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However, due to this very damage bonus you will spend less than 30% of the time fighting the last 30% of mob's HP. Most people treat Ruthless as a 40% damage bonus that's active 30% of the time.
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