Too many variables involved to calculate the exact potency. Are you using Selene? Do you have high skillspeed? Are do you have latency? Do you clip GCDs anywhere in the rotation slightly (like I do). All of these affect the BFB placements, a lot.

We can estimate it as a 0.9375% (or *1.009375) average DPS increase of 8GCD vs 9GCD. It's a very small increase, noticeable on paper. Not in game.