The pure cut-off where Thunder is better than B1 is 6s left on any Thunder or less, in terms of potency.

30 initial + 40 per tick, 4 ticks = 160 + 30 = 190.

However, pure potency isn't all there is, it's also about getting that chance for Thundercloud, and my personal cut-off is 9s, or basically one more tick of clipping.