Because mechanics can really throw a wrench in your rotation, I find the best method is to be quick on your fingers and use the rule of 3. Probably more important to learn this than to learn an exact rotation.
6+ seconds remaining, you're good for Fire IV
3+ seconds, Fire I
<3 seconds, swiftcast Fire I or use a Firestarter proc, or if necessary Transpose.
And if you're mp is too low for another Fire IV just go straight to Blizzard III, which in AF and with enough SpS and a good ping can even be cast at 1.5 seconds.