How is AD, and MR calculated?

% damage reduction = Resistance/(100+Resistance)

For example, 50 Armor or MR grants 50/(100+50) = 33% damage reduction. 100 Armor/MR grants 100/(100+100) = 50% reduction 200 Armor/MR grants 200/(100+200) = 66% reduction

Is that what you were looking for?

Defense = ( (Starting amount / 100) * (100 - % pen) ) - flat pen

So starting at 100 MR with a void and 10 mpen is ((100/100 ) * (100 - 35) ) - 10, or (1 * 65) - 10 or 55.

Defense reduction is then calculated as 100/(100 + value) or 100/(155) or .645

This is then multiplied by the incoming damage to be AD * .645 (for example) = total damage.

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To shove it into one long formula if that is what you are looking for:

Total Damage = X * ( 100 / ( 100 + ( ( Y / 100 ) * ( 100 - Z ) ) - V) )

Where: X = Incoming Damage Y = Appropriate Defense Z = % Defense Reduction V = Flat Defense Reduction

I believe that some % damage reduction happens after flat damage reduction in which case you would change ( 100 + ( ( Y / 100 ) * ( 100 - Z ) ) - V) to ( ( 100 + ( ( Y / 100 ) * ( 100 - Z ) ) - V) * F)

Where Z = % Damage Reduction before Flat Reduction and H = % Damage Reduction after Flat Reduction

Hope this helps!

You can also think of 1 point for armor/MR makes you have 1% more effective health against that damage type.

So 100 armor is 100% increase to effective hitpoints against physical damage - which equates to 50% physical damage reduction.