Armor Penetration
Armor Penetration Tooltip = 35.72*ArmorPenetration^1.88/(LevelConstant+ArmorPenetration^1.88)
Targets Mitigation % = Mitigation-StatBonus-ArmorPenetrationTooltip*(1+FeatBonus)
...where the level constant for level 60 is between 1225377 and 1225660.
Your % of Armor Penetration subtracts directly from the targets mitigation to a minimum of 0 mitigation. Abilities effects stack, are applied after Armor Penetration and can reduce the targets mitigation into negative numbers. See PvE mitigation info in post #2.
Comments
Armor Penetration Tooltip = 35.72*ArmorPenetration^1.88/(LevelConstant+ArmorPenetration^1.88)
Targets Mitigation % = Mitigation-StatBonus-ArmorPenetrationTooltip*(1+FeatBonus)
...where the level constant for level 60 is between 1225377 and 1225660.
Your % of Armor Penetration subtracts directly from the targets mitigation to a minimum of 0 mitigation. Abilities effects stack, are applied after Armor Penetration and can reduce the targets mitigation into negative numbers. See PvE mitigation info in post #2.
All credit to freehugs9 http://nw-forum.perfectworld.com/showthread.php?301761-Calculating-stats-and-their-effects
2540 is 24%, and that's the most you currently need in PVE.