Keep in mind however that it will take a few seconds to fire off this opening salvo (fire FS, 1sec for GCD, then cast time of lava bust) and most opponents are going to be running around with 18-24k HP, plus any HoTs they have rolling on them will mitigate some of this damage. In those 2-3.5 seconds it took you to fire off this salvo, what are they doing?

The damage will also be a bit more variable, as there are ranges to the various spells and crits. Some may have auras/gear/etc that also provide some resistance to fire damage so that would also have to be taken into consideration.