It depends. If you don't get a proc you really can't do a Swiftcast F4 ... you'll run out of Astral Fire (F3P > F4 F4 > F1 (10s) > F4 F4 (5s left) Swiftcast F4 F4 (no AF left) B3 will fail terribly).
If you get a proc, it's still somewhat iffy. You have to go by pot/Trick Attack timers too, getting any of those makes Flare stronger than F4. A standard pure potency upgrade with a proc is F3P > F4 F4 F1 F4 F3P Swift F4 Convert F4 F4, which is fine MP wise and AF/Enochian wise, but it may not sync up with your pot/TA in such the way that F3P > F4 F4 F1 F4 F4 F3P Swiftcast Flare Convert F4 B3 does. We're talking min/maxing here, it's really not even a big enough deal to care or change your parse in a way that anyone would notice.
Me I just do the Swiftcast Flare usually because of that, even with the proc, because my personal pot and TA timers make that stronger - which may vary BLM to BLM. What I WILL adapt to though, is if I got an MP tick before combat started ... if you get the MP tick and the F3P, you can do F3P > F4 F4 F1 F4 F4 F3P Swiftcast F4 Convert F4, which is as lenient as it gets to interacting with TA/pot because you can put your Swiftcast in one of two places if you wish.
