I've started levlling a BLM. Running the lower dungeons I'll usually throw a Sleep, or Thunder + Sleep on a couple enemies then start just trying to do as much damage as possible on the tanks target.

With the attack Skills available in, say, Sastasha (Fire I, Blizzard I, Thunder I, Scathe), what rotation would you recommend? Currently I start out with:
Fire I until MP gets really low
Transpose
Thunder I (if not up already (not usually at this stage))
What I then can't decide is whether to:
a. Use Blizzard till the transpose is available again
b. Just Start casting fire (first will be weak / cancel Umbral Ice, second will be mediocre with neither Astral fire or Umbral ice, third and beyond will be Strong with Astral Fire)
c. Use scathe until the MP fills and the Umbral Ice wears off, then cast fire (from no buff) continued by fire with Astral Fire buff
Please give reasons of figures with your answers if possible.

I don't have a parser or know how to set that up, so it's tricky to be able to work out which rotation is best.