I actually redid the math, and with 3 GCDs with bane up you get 2.5 ticks, which is 275 potency.

Adding it to the triple ruin damage, bane will beat out fester even at 1 target, at 515 vs 410 potency. However this ONLY works if you bane a full rack of DoTs.

Xero you forgot that with bane you get the dot ticks going while you ruin.