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Okay, I've been looking up Armor Penetration data from wowwiki to elitistjerks to wowhead, wanting to get an idea how much extra damage 280 Armor Penetration (ArP hereinafter) will generate. I picked 280 ArP because that's the value from dualing DoBM's.

I've worked on existing spreadsheet downloaded from elitist jerks and modified it so for this purpose.

Please help me identify if my calculations are right on spot or not.

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Formulas used:

Formula on damage reduction is based on this post in Elitist Jerks:
http://elitistjerks.com/f31/t16629-raid_boss_armor_values/p2/

Damage Reduction % = 100*(armor)/(armor-22167,5 +467,5*Boss level)

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Boss armor value:

List of level 73 Boss armor values can be found here:
http://elitistjerks.com/f31/t16629-raid_boss_armor_values/

I'm going to use boss armor value of 6200 (low end) and 7700 (high end) to give some ideas on how much damage increase 280 ArP will be giving.

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Raid armor debuff legend:

Faeire Fire (FF): 610 total
Sunder x5 (SD): 2600 total
Curse of Recklessness (CR): 800

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Default damage reduction on bosses with 6200 and 7700 armor:

7700, Damage Reduction: 39.17%
6200, Damage Reduction: 34.14%

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Calculation assuming *NO* armor debuff casted on the boss:

Boss with 7700 armor -
Boss Armor after 280 ArP:
7420, Damage Reduction: 38.29%
Gain in damage:
0.88%

Boss with 6200 armor -
Boss Armor after 280 ArP:
5920, Damage Reduction: 33.11%
Gain in damage:
1.03%

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Calculation assuming *FULL* armor debuff (FF+SD+CR) casted on the boss:

Boss with 7700 armor -
Boss Armor after full debuff:
3690, Damage Reduction: 23.58%
Gain in damage:
15.59%
Boss Armor after full debuff + 280 ArP:
3410, Damage Reduction: 22.19%
Gain in damage:
16.98%
Difference in damage:
280 ArP will generate 1.39% extra damage.

Boss with 6200 armor -
Boss Armor after full debuff:
2190, Damage Reduction: 15.48%
Gain in damage:
18.66%
Boss Armor after full debuff + 280 ArP:
2190, Damage Reduction: 13.77%
Gain in damage:
20.37%
Difference in damage:
280 ArP will generate 1.71% extra damage.

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Conclusion:

With no raid debuff, 280 ArP will generate 0.88%-1.03% extra damage on a level 73 boss armor ranged from 6200-7700.

Under full raid debuff, 280 ArP will generate 1.39%-1.71% extra damage on a level 73 boss armor ranged from 6200-7700.

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Remarks:

Another spreadsheet found here:
http://elitistjerks.com/f31/t25877-armor_penetration_arp_spreadsheet/

Have listed that, under full raid debuff, a level 73 boss, 275 ArP will generate 1.97% more damage.

The formula this spreadsheet is using is:

Damage Reduction% = (Boss armor / (Boss armor+10557.5)) x 100

This formula, when used on a boss with 7685, yields a damage reduction % of 42.13, which is greater than the one using the other formula posted above, 39.17% on a boss with 7700 armor.

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(с) Solos
http://www.wowarmory.com/character-sheet.xml?r=Nagrand&n=Solos