My BLM DPS seems lower than it should be

[Rongway]

Thundercloud makes your thunder spells do their initial damage plus all their DoT at once, and then you still get the actual DoT. So for Thunder III, you get 70 initial, plus 8 ticks of 40 all at once, for a total initial hit of 390. Then, you get 8 ticks at 40 potency, for another 320 if it lasts the whole duration. A full duration Thundercloud III is 710 potency. But if you only get three ticks of it, it's the 390 initial hit plus 120 total DoT, or 510. This is more than the potency of an Astral Fire III buffed Fire IV, so as long as you get three ticks out of it, it's worth dropping Fire IV in favor of Thundercloud III.

Since all thunder spells tick for the same amount, you won't effectively increase the damage unless reapplying thunder would extend the duration by at least nine seconds.