Whiskeybravo, the way to calculate the additional dmg is to multiple by the potency by the percentage buff. Example:

300 x 15% = 45 thus 300 + 45 = 345 potency
Or
300 x 85% = 255, subtract that from 300 (300-255=45)

When you divide 300 by .85 you're finding the number that 300 is 85% of... Or 352.94 x .85 = 300. This is NOT the correct way to find percentage buffs

As well the percentages are multiplicative, meaning with HT your 100 potency skill is hitting for 115. After disembowel (10% buff) the new potency is 126.5 (115 x 10%) instead of 125. This also applies to B4B. Best to go back and read the first page Avyar breaks it all down. I'm doing a terrible explanation.