Poison II / DoT Spells:
I tested DoTs mostly using Poison II and with 2 different methods. The first mimicked the way I originally tested DoTs prior to 1.19
here; the other was using star marmots outside Ul'Dah, which have a low enough max HP for me to directly figure out by throwing rocks on a DoH class.
Using the Lemur "Soil Smear" method by using a DoT then having the Lemur give the enfeeble back to me, I was able to determine that the amount of damage per tick. PIE did seem to affect the damage per tick. Briefly, I took 2 THMs, one with PIE of 235 and another with 271 vs. Lv42 Lemur. Using Poison II, the set with 235 PIE gave 28 Damage per Tick, whereas the build with 271 PIE gave 29. The damage also seems to vary greatly on elemental resist now. For instance, taking a R50 CON with THM trait on a Lv44 Lemur:
Choke II: 16 | Burn II: 7 | Poison II: 23 | Bio II: 12
This suggested to me that elemental resists may play a role in determining the damage per tick. I then tried to confirm this by giving myself Shell II, then having the Lemur use "Light Scratch" to steal my Shell II, then using the same Soil Smear technique. The result showed that for a THM with a 25 damage per tick Poison II (261 PIE vs. Lv42 Lemur), this value was reduced to 21 damage per tick whent he Lemur had stolen my Shell II, confirming that elemental resistances do factor into the damage determination formula.
Moving on to Star Marmots in Ul'dah, which I found to have an HP of exactly 67 by throwing rocks at them on DoH. I would then extend the max HP bar on the UI to across the whole screen then use DoTs to see the HP drop off. I would then screenshot and very crudely count the # of pixels taken off the HP bar and relate it back as a % of the total HP bar to find the % of 67 HP taken off. Doing this with Poison II at both PIE = 220 and 301, I found that the Poison II damage per tick appeared to be exactly 30 HP/tick for BOTH builds, implying it caps.
* Damage per tick affected by both user PIE/enfeebling and the mob's elemental resist
* The damage per tick on spells appears to have a cap (30 for Poison II)
Admittedly, these tests were done rather crudely. For instance, I did not even note my actual enfeebling magic skill during these tests, only the PIE change. Overall I was extremely disappointed with DoTs (especially at the very low Poison II cap...), which probably led to the lack-luster testing and lack of a formula determination. This would be an interesting set of tests to revisit when we are able to re-allocate stats, but for now, I think SE has killed a DoT build (which was admittedly overpowered prior to 1.19) for the time being with the very low caps. Of note I do not know what a Poison II would deal to Ifrit since this is a difficult topic to test on high level mobs without knowing their elemental resists; so I am unable to answer the question of "should I use DoTs on Ifrit?"