BLM

[HoroBoro]

"Grants a 5% chance that after each damage over time tick inflicted by any Thunder spell, the next Thunder, Thunder II, or Thunder III will add its full damage over time amount to its initial damage, have no cast time, and cost no MP."

I don't see anywhere in the tooltip that it states a set base potency on proc. Due to not stating a base potency, the clear choice here is the highest one.. which would be Thunder III.

Keep in mind when you overlap thunder debuffs they do not merge, you will lose the dps on the remaining ticks from the "refresh."

You forgot to bold one extra word. It makes a big difference. See: here

Read Descriptions for Thunder and friends. Specifically states that, if you cast Thunder I, you will do a 240 potency for the Thundercloud even if you use Thunder 3. I haven't done tests but, I see no reason for tooltips to not be telling the truth.