The age-old question. It's been bugging me for a while, ever since an ex-guildy mentioned that it was no longer best to swap FM with your fellow mages. I didn't believe him at first (mostly because I had just met him and didn't trust him to give me advice yet) but because I would probably make my brain explode if I tried to do the calculations myself I went and researched the idea, just in case. And it turns out he was right.
I went to the usual sites. Wow.com, Wowwiki (which was surprisingly a step up from useless as it hasn't been updated for about a year), the official Wow forums, and I even waded through Elitistjerks. God I hate that site. It's so hard to find anything useful. And yet I love it at the same time because you can almost always trust the information you find there. I struck gold at MMO-Champion. Before you read that, understand that it REALLY gets into the nitty gritty of things. Most mages could happily go about their way completely ignoring it. In fact, it really only applies when you're looking at high-end ICC 25 raiding, when everyone is in full iLevel 264 gear or higher.
Another link I found to back that one up (constructed differently but with mostly the same priority list) was on the official forums, here.
So after my research I've come up with a checklist for FM targetting that I'm going to follow from now on.
1. What is the target's usual DPS output? If this information is not available, what is the target's gear score?
A higher gearscore means a higher spell damage value, which means if they do crit, it will be for a bigger number. It also means in some class' case that they have a 2- or 4-set Tier 10 bonus, and without delving into each specific class' specific bonuses, this usually increases haste, crit or spell damage, all of which are good for my purposes.
2. Which class has a higher crit chance?
This means I will crit more. If I'm pulling decent DPS and gain a good benefit from more crit chance then this is of benefit to the raid.
3. Which class will gain more benefit from having a higher crit rating?
This means that when they crit, they'll crit for more, which will benefit the raid.
I would almost add that if I'm specced fire at the time, I would put my own increased crit chance before others. Whereas if I was specced arcane, I would try and aim to benefit others more than myself because arcane just doesn't get as much return from crit.
I'm curious to know what other mages think. Do you also run through a checklist? Or do you just put your FM on the nearest other mage?