I actually forked up the calculation there, mixing up the amount of damage taken with the actual DR. It’s 58% when you have Barkskin active (which is huge compared to 42%).
It’s multiplicative. Basically every source of DR is multiplicative in the game.
Damage taken: DMG = 100
Damage reduction, source 1: DR1 = 30%
Damage reduction, source 2: DR2 = 40%
ActualDamage = DMG*(1-DR1)*(1-DR2)
100*(1-0.3)*(1-0.4) = 42 damage taken
Actual DR = 42-100=58% DR
If it was additive it would look like this:
ActualDamage = DMG*(1-DR1-DR2)
100*(1-0.3-0.4) = 40 damage taken
Actual DR = 40-100 = 60% DR
There’s probably a smarter way to calculate it, but that’s how I’ve done it.