It's been a long time since I've been fire on my raiding mage, but I think the old rule of thumb was to get a full stack of scorches whenever you expect a mob to last longer than 15secs. Otherwise you'd just want to do fireballs. Also your lack of spell hit is going to hurt alot more as fire as opposed to arcane, you are probably getting alot of resists. Spell hit cap for arcane is 6% (72 rating i think), the cap for fire is 13%(164 rating), frost is 10%(around 124 rating).