How armor and damage reduction interact

[Solu9]

How does the calculation look like when counting both armor and damage reduction from fx. Elune's Grace?

We can make an example.

Armor grants 35% damage reduction
Elune's Grace grants 20% damage reduction

Does that mean 55% damage reduction after the calculation?

 

[Wardonis]

test it i'd suggest. make a unit set his base attack to 100 and his rolls to 1/1 and see how much damage he does per hit on a target

 

[afisakov]

Pretty sure it isn't additive stacking.
At best it is multiplicative.

While a test is best to confirm this, the way it should stack is:
armor 35% reduction allows 65% through
elune's grace 20% reduction allows 80% through
together they allow 0.65*0.8=0.52 through
therefore the combined effect is 48% reduction.

btw, when you run the test don't forget to give attacker chaos attack type and defender 0 health regen for clean results.
Don't want armor class (heavy,medium etc.) or hp regen obscuring the results.