Guild Wars 2

What I mean, is on runes like the Rune of Durability (PvP) the 2pc bonus is +5% boon duration and the 6pc bonus is +15% boon duration. Does this come out as baseDuration*120% or is it (baseDuration*105%)*115% ?

The best way to find out how much it actually gives, is by opening your hero panel and checking your stats, just look at the boon duration stat at the right side. However, I can tell you that all percentage changes to stats and durations are always relevant to the base stat, so its simple changes, not compound. (As in, additive, as you put it.)

-Mair

It’s additive. So in total you get +20% Boon Duration from Superior Runes of Durability.