Combustion did spread seem to spread from his body to legs, but only seemed to spread to a single leg (regardless of how many were up), never consistently the same leg either.

i suggest dots spread out from body only to forelegs, but from any legs to body

Assuming you have perfect latency, and you wait for living bomb to drop off before refreshing, you are most likely still going to miss .5-2 seconds (from current cast) at least, which means you WILL miss ticks. I use the 'you can detonate with 3 seconds left' as a safety net situation. Once I stop casting and my living bomb is within that range, I reapply.

As for whether it is worth it? Just look at the damage numbers. Living bomb explodes for 1395 +104.5% sp (wowhead), fireball is ~1500 (average) +150%sp. Fireball takes ~2x the cast time (at a reasonable haste level). Clearly, the LB explosion is worth more than half the fireball. The only other complication I can think of is ignite (~15% of fireball dmg over 4 sec), which LB does not proc. But even with that included, LB comes out ahead (significantly).

I would say it is ideal to refresh LB asap - and in doing so, maximise the explosions - but not at the expense of cancelling a spell cast (obviously).

I came across a spread sheet with the haste break points for mages. Since it's not published here yet I figured I'd share it. It says that for a non goblin, not using frost armor, with the 5% raid buff wants 3036 haste for living bomb, 3056 for nether temptest, 3056 for glyphed combustion, and 4052 for unglyhphed combustion. The link includes every other variation too.

https://docs.google.com/spreadsheet/...ZmFYa0E#gid=39

When do you get into this position? Assuming you follow the rotation in the OP this shouldn't happen? You cast fireball to fill, and IB when you get HU (refreshing LB in between). The only scenario you could get HU and Pyro! at the same time is when
Fireball 1 cast
Fireball 2 cast
Fireball 1 lands (crit) - HU
Fireball 2 cast ends
Inferno blast cast (crit) - Pyro!
Fireball 2 lands (crit) - Pyro! + HU!

Obviously the next step here, as in the OP rotation, is to use Pyro! You can't use IB as it is on CD.

The only other scenario I can see this happening is as you cast combustion (as the initial damage can hit/crit, causing HU to proc Pyro! or lose HU). In this case you could feasibly get Pyro! and HU at the same time as IB is off CD (as combustion resets this CD). In this case, use Pyro and resume normal rotation (or use IB if multiple targets, and resume normal rotation - you will lose a single Pyro! proc).

Is there another situation this can occur?

If refreshing LB asap, over the fight you'll cast less FB and more LB. however LB doesn't proc HU or HS, so refreshing LB asap will provide less pyro proc causing a small loss of dps. The gain of the LB explosion count seems not worth it.

The best next step is to cast FB + pyro ! : they will land approx at the same time so if only one of them crit your HU proc will become HS.

I think the "...then Pyro" in step #4 is redundant, and should be left out. There should be one step per line. The Pyro! proc you get from casting IB when HU procs is caught later in the priority list so it's not needed on line #4.

Steps #5-7 just emphasize that refreshing LB is a higher priority than casting pyroblast unless you have both Pyro! and HU, in which case you use up the Pyro! first, hit LB, now you have a chance your Pyroblast crit, to use another Pyro! proc, otherwise force it with an IB and then Pyroblast, then continue.

At least, that's how I read the list, and how I've been executing it in practice.

Edit: Removing the "then pyro" unfortunately causes the case of your current FB critting and IB (with HU up) critting, generating Pyro! + HU case, with LB about to expire, as Pyroblast first, then LB, which should be LB first, then Pyro, to ensure Pyromaniac buff does not expire as you're casting the Pyroblast. Some better language should be employed to reflect this border case. Or common sense should be used, but I may be expecting too much ...

So if I understand this correct we will be going to use unglyphed Combustion as soon as we hit 4052 haste (as nongoblins). Because that would give us 12 ticks on combustion -> 24 in the time of a glyphed combustion while glyphed combustion would just give us 23 ticks because you need 5036 haste to get this 24th tick. I have no clue on the math behind this and I don't want to question anything behind this but I think this looks kinda weird. Still, if true would be interesting.

It isn't just the explosion you are after though. As I mentioned, use the few seconds to proc an explosion as a safety net. If you are routinely waiting for LB to finish you are going to miss a few ticks of living bomb as well as fewer explosions over the fight. You only refresh LB ASAP where you happen to have just finished a cast as the LB is able to explode if refreshed - an incredibly rare situation.

More often than not you find yourself refreshing LB with 0-2 seconds remaining I found.

LB proccing HS should be irrelevant, since, as I said, you only use that 'proc ASAP' as a safety net. Obviously if you have HU up you use IB before refreshing LB. Yes LB cannot proc HU, but I feel the additional LB ticks + explosions make up for that.

RE Glyph of Combustion
Does Glyph of Combustion not also double the duration of the DoT ?