Quote Originally Posted by HoroBoro View Post
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.
The tooltips for the individual Thunder spells are inaccurate. Or rather, you would want to focus on the tooltip for thundercloud:

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.

If you test it in-game, you would find that it actually holds true.

When you get a thundercloud proc, if you mouseover the tooltips, you would see that the first line of the tooltip changes to Deals lightning damage with a potency of 240/295/340, respectively for T1, T2 and T3.

Further testing on dummies also shows that a Thunder III thundercloud proc does more damage than a Thunder thundercloud proc. In my case, Thunder III does around ~750 dmg, whereas Thunder only does ~500 dmg.