How Can I Apply DoT's Most Effectively?

I have a BLM at lvl43, and a Monk at lvl33, and during the low level dungeons while there's never an issue dishing out the needed damage, I'm always wandering what the hell I'm doing with DoT's, particularly on packs of Mobs / adds which I find more confusing than a single target with lot's of health (aka training dummy).

Things I've been considering in particular:

• If the amount of ticks (before the mob actually dies) + initial damage of the DoT skill are gonna be less than just doing my straight damage skill, there's no point applying it.
• For Monk - With the above in mind, do I DoT (Touch of Death) the first enemy immediately, or DoT the next after I've gained some damage buffs (greased lightning / twin snakes)?
• With BLM do I DoT (thunder) each and every enemy at the start of the pull, or am I wasting too much time? After all, the first mob could essentially be dead by the time I've dotted 5 enemies.
• With that in mind, can the group do more damage if I wipe out single targets quickly in succession, instead of just considering my own dps through DoT's on each enemy and aoe's? What I mean is, by reducing no of mobs up quickly, I'm reducing damage on tank and thereby freeing the tank and healer up to be able to dish some dps out. Maybe it's my imagination, but sometimes it feels much faster to sleep the pack and have all 4 players working quickly together through each numbered mob.
• With Monk so far I have Demolish, Touch of Death and Fracture (from WAR). How do I work all that into a rotation without losing buffs like twin-snakes and greased lightning? Again for groups, how to distribute multiple types of dot's (surely it's a waste to chuck up 3 on the first mob which is killed before half the ticks can tick)?
• With Monk - Would it be better to do bootshine > twin-snakes (dmg buff) > Demolish (DoT), then go into the pure damage rotation (ending in Snap Punch), or instead of ending with snap punch, to throw a bootshine (1st Mob) > true strike (1st mob) > Demolish (2rd Mob), then bootshine > twin-snakes (reapply dmgbuff) > Demolish 3rd mob?
• BLM has three levels of thunder. Why? Isn't Thunder 3 just always better?

Anybody got some tips on this, or able to point me at a good guide that specifically cover's using DoT's well?

On multiple targets, I'd imagine it's way more profitable to AoE them with Fire II than to try to DoT any of them. Certainly at level 50, BLM is the dps job with far and away the most ridiculous AoE damage between Fire II and Flare.

Overall dps will be higher when AoEing multiple targets. Depending on party composition and gear, you may want to sleep stuff because it's safer. Single target focusing certainly kills individual monsters faster, but shouldn't be faster overall, esp. with BLM AoE potential.

I can't really speak to MNK, but I agree DoTs are only useful if the target will live long enough to experience the entire DoT. Only potential exception to this is SMN.

I can answer some of the questions related to BLM.

If you have more than 3 enemies, you'll do more total damage per skill if you focus on AOE attacks and don't bother with DoTs. The only thing DoTing more enemies up gets you is more chances for Thundercloud, which is good damage, but completely up to RNG.

When making the decision on whether to AOE or focus on single targets, it depends on how hard you want your healer to work and for how long as well as your party composition. If all the enemies are alive until the end, then the healer has to work for the entire fight and cannot DPS. If you're paired with another BLM or a BRD with strong AOE, go buck wild and AOE everything. If you're paired with a DRG or a MNK, who can AOE but it's lacking, then you can focus down one monster at a time. It also depends on your tank. If your tank cannot hold threat on more than one monster (maybe he's undergeared), then hold back a little.

Finally, regarding Thunder 1/2/3, the only difference is mana cost, casting time, and duration. Initial potency and individual DoT tick potency is equal. In order to decide, you need to look at how long you spend in Astral Fire and Umbral Ice. How long does it take for you to get to full MP during Umbral Ice? For me, casting Thunder 1 isn't enough time for me to get to full MP, but Thunder 3 keeps me casting for an extra MP regen tick or two. Therefore, I can cast Thunder 2 and be timed perfectly to resume my Fire rotation as soon as my MP fills up. Also, how long am I in Astral Fire? For me again, Thunder 1 falls off before I've run out of MP, so I've lost damage, but Thunder 3 ends up having 7-9 seconds left when I run out, forcing me to clip the duration and lose damage again. Therefore, Thunder 2 is still the best option for me. There are three exceptions. Occasionally, after I switch to Umbral Ice, I only have enough MP for Thunder 1. No matter what, it'll be a DPS loss to wait for an MP regen tick (ABC = Always Be Casting), so cast Thunder 1 just to get the DoT going even though it's going to fall off before I re-apply it. Now that I think about it, it'd be much better to use Convert before switching to Umbral Ice, casting a few more Fire 1, and having enough for Thunder 2 after the switch to Umbral Ice. The second case is the pull, I usually start with Thunder 3 and try to time it so it lands just as the tank pulls. The last case is Thundercloud. Always use Thunder 3 when Thundercloud comes up.

Thank you both for the suggestions! Very helpful :)