You just prorate the tick, so if its 0.5 seconds till GCD from the application you just say its 0.5/3*potency.

I'm a bit confused by what Xero put down but I understand the idea of it.

Ideally bane saves you 3 GCDs that are used to apply dots.

Applying dots during those 3 GCDs gives you what is approximately 35 (1 Bio2 tick) + 20 (miasma application) + 110*0.5 (roughly all dots applied at 6 sec, we stop at 7.5 sec) = 110 potency

3 Ruins instead (because of bane application) gives you 240 pot, and you're saying 410 > 240 for single target. Mana costs and ramp aside, this makes sense and for double target bane is never worth it.