(First-time poster, longtime Black Mage!)
I feel like I should point out that it's actually not a bad thing to use an Umbral Heart on a Fire instead of a Fire IV. Umbral Hearts don't affect damage or cast time, they just nullify the MP multiplier imposed by Astral Fire; since Fire costs more MP than Fire IV, spending a heart on Fire actually saves you more MP! Most of the time this won't really matter, but at certain tiers, it can mean the difference between continuing to cast and having to wait for a tick. (This seemed to be the case with the eight-F4 opener for me at 60; I split them 2-3-3 instead of 3-3-2 to get the Fire into a Heart slot.) If we accept that it's okay to use an Umbral Heart on Fire as well as Fire IV, we can solve a lot of the problems people are having in this thread with timing and proc placement.
For example: if Sharpcast is up (which it should be every other round), we can use F4x2 > Sharp F1 > F4x2 > Proc F3 > F4x2 instead of sticking the proc in haphazardly. Dividing the Fire IVs into three blocks (2-2-2) instead of two (3-3) gives us a lot more room for movement and Enochian renewal, as well as providing room to slip in a Thundercloud (or a Foul, if you're holding it) after any two Fire IVs without risking our timer. If we don't have Sharpcast up, it's still mathematically advantageous to use a second Fire to make a 2-2-2 grouping if we have a Thundercloud (or Foul) in our pocket. The 3-3 grouping is still our best go-to without any procs or Foul, but if there's a lot of movement going on, it might be worth using a 2-2-2 grouping instead (it's a slight damage loss, but not nearly as bad as dropping Enochian).
TL;DR: it doesn't matter what you use your Umbral Hearts on in single target, so space out your Fire IVs if you've got procs to use!
My cheat sheet (feedback is welcome if I made errors somewhere!):
Code:Opener:
B3 > Eno+T3 > LL+B4 > F3 > (Diversion+)F4 > F4 > (TC) > Sharp+F1 > F4 > F4 > F4 > Proc F3 > Convert+F4 > F4 > F4 > back to UI (with Foul)
(Note 1: I use Ley Lines between Thunder III and Blizzard IV because that's the earliest point it can be used that will encompass all eight Fire IVs.)
(Note 2: If you happen to get a Thundercloud proc by the second Fire IV, use it! The initial cast will have ticked three times by then, which makes the clipping worth it.)
If Sharpcast is up:
F3 > F4 > F4 > (TC/Foul) > Sharp+F1 > F4 > F4 > (TC/Foul) > Proc F3 > F4 > F4 > (TC/Foul) > back to UI
If Thundercloud procs before the second Fire IV, or if holding Foul:
F3 > F4 > F4 > (TC/Foul) > Sharp+F1 > F4 > F4 > (TC/Foul) > F1 > F4 > F4 > (TC/Foul) > back to UI
(Note 3: if Thundercloud procs after the second Fire IV, you're better off holding it until its usual spot in Umbral Ice, unless it would fall off just before.)
If Firestarter procs without Sharpcast:
F3 > F4 > F4 > F4 > F1 > F4 > F4 > Proc F3 > F4 > (TC) > back to UI
Otherwise:
F3 > F4 > F4 > F4 > F1 > F4 > F4 > F4 > back to UI
