As BLM I'd agree with above.
basic 50-60 rotation is B3,T3,F3,F1,F1,F1,F1 Repeat.
B3 puts you in unbral ice and increases Mana regen. T3 should get you to max mana before hopping back to Asral fire with F3 and spamming F1 till out of mana.

If doing AOE damage you can go with
B3, T2, F3, F2, F2, Flare, Transpose/Convert repeat
Tranpose will give you a small boost to mp when putting you in Umbral ice as Flare will use all mp so you won be able to switch with B3.
If you use convert you will still be in Astral fire after flare as it just takes 20% hp for mp so will need to B3 after Convert to begin mana regen with B3 and T3 again.
Transpose will switch you to Umbral ice after flare and give you a small mp boost enough to fire off B3 or T3 to max you out again.

Obviously in between all that hit any procs you get depending on group or single target for Thunder.
Have fun though, its a great job and when you get your Leylines just fire them down at the start of your rotation.