Maybe it's static and you can multiply the number of stacks with sth.
*errr* you don't.
http://cata.wowhead.com/spell=84839
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Each time you take damage, you gain 5% of the damage taken as attack power, up to a maximum of 10% of your health.
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Maybe UnitAura has a new value as result. You could print all the UnitAura values and see if there is anything valuable in there.