DPS parsing Black Mage numbers since patch

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Personally, I use Thunder III as part of my rotation. The new mana tick is unreliable and Thunder III is my current solution, heck I even use it on Titan EX and still do very well.

I have 251 piety, and after doing 5 fire 1s I never have enough mana to do thunder 3. How do you wind up with enough mana to use thunder 3 without only doing 4 fire cast? Thunder 2 requires me to wait or use blizzard after. If I use blizzard after it would have been better to use thunder 1. Am I wrong on this?