What is the formula for armor/magic resist reduction?
As far as I can tell, it's something like this:
damage reduction = armor / (armor + 100)
Is this accurate?
Yes, that's correct. The calculations are:
Armour Resist: Damage Reduction = Total Armor / (100 + Total Armor)
Magic Resist: Damage Reduction = Total Magic Resistance / (100 + Total Magic Resistance)