This is a pretty good rotation, it's extremely high in WoTLK Gold dps as well as efficient. It's also said that if you're low on mana levels, go to a more conservative rotation where you use the missile barrage function at three stacks, or lower.

So while you can follow this more easy approach, a better option is to get a feel of your raid's environment and how long a fight is before you plan for what you'd like the is the right number to be.

For instance, where you'll draw the line in searching for missile barrage weapons and having to hard cast and drop Arcane Missiles your pile because, trust me that it's possible to get lucky and at Forest Axe, you'll have to constantly be casting arcane blasts and hoping for a missile barrage Croc and it might be six 810 Casts before you're there if you get unlucky cheap WoTLK Gold
 and then you'll lose your entire mana bar.