My understanding is that all forms of damage reduction including stacks from APDs and Legacy of Nightmares work multiplicatively - so your total damage reduction formula is 1 - (1-reduce1) * (1-reduce2) * (1-reduce3) * ...
For example 2 separate 50% damage reduction modifiers will not make you immune, it will make you take 75% less damage.
So every stack of APD means you take 12% less damage then the previous stack. 8 stacks would be 1 - (1-.12)^8 = 1 - 0.88^8 ~ 64.04% damage reduction. This is turn is multiplied by any damage reduction from armor, resistances, unity etc. Full immunity is never possible by stacking reduce damage % buffs.
If you need to farm any hellfire amulets you can use FS for those. Although good news is if you are concerned about the stash space the FS take up that won't be a problem next patch.
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My understanding is that all forms of damage reduction including stacks from APDs and Legacy of Nightmares work multiplicatively - so your total damage reduction formula is 1 - (1-reduce1) * (1-reduce2) * (1-reduce3) * ...
For example 2 separate 50% damage reduction modifiers will not make you immune, it will make you take 75% less damage.
So every stack of APD means you take 12% less damage then the previous stack. 8 stacks would be 1 - (1-.12)^8 = 1 - 0.88^8 ~ 64.04% damage reduction. This is turn is multiplied by any damage reduction from armor, resistances, unity etc. Full immunity is never possible by stacking reduce damage % buffs.
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If you need to farm any hellfire amulets you can use FS for those. Although good news is if you are concerned about the stash space the FS take up that won't be a problem next patch.