if you are in decent gear (no greens) you should be oding 4 arcane blasts before arcane missiles. Ideally you would keep casting blast until the hasted missile procs, but you have a ~90% chance of it proccing once you get to the 4th cast of arcane blast anyway. That old macro of mine was before a recent patch where they buffed the stacking buff you get from arcane blast to have 4 stacks and cost less mana. Hell just straight casting arcane blast is theoretical best dps provided you have the mana for it.

also, at the end of my macro I added

/use mana sapphire
/cast mirror image

and that can help boost DPS (moreso back when we had T8 set bonus)