Hey Rain_clouud you may want to change it just a bit. I find it better to start out with Fire 3 to get astral III then fire til proc. Say you've cast Fire 1 x 3 before Fire 3 procs, then you've had to waste three casts to hit Astral 3.

Solo I usually go : Thunder > Fire III > Fire 1 until proc or OOM > Blizzard III > Thunder > Fire 3 > repeat. I'm usually back up to full mana by the time the Thunder finishes being cast.

In Instances on trash I don't even bother with Thunder until the first rotation is done: Fire III > Fire 1 until proc or OOM > Blizzard III > Thunder > Fire III repeat.