BLM

[Thyrllan]

If you read the tooltips for Thunder I - III you would see that the potency increases with each rank that is hard cast to trigger Thundercloud.

The tooltip is flawed.

Thunder I -> Thundercloud proc my Thunder III hits for avg 711, low 691, high 739
Thunder III -> Thundercloud proc my Thunder III hits for avg 706, low 696, high 735

Differing numbers, but not large enough to be worth 100 potency. Thundercloud's potency is dependent on which Thunder you spend the proc on. For fun I hardcasted TIII and used the proc on TI, it hit for 495