I do believe there is no reason to use Sharpcasft Fire 1 in between rotations DPS wise. You gain more betting on the 40% and having a free Thundercloud proc worth 780 Potency after that Fire 1. This mean it should do about 660 Potency if you clip 3 ticks. 700 Potency if you clip 2 ticks. It's much better than Fire 3 proc. Actually, F3 proc is 432 Potency and T3 Thundercloud is 460 Potency + 320 from ticks lol.