After testing I've drawn some conclusions about how the damage over time effects are calculated.
Applying acid burn / irradiate deals an invisible 0 damage. This is exposed by having Rally/300 percenter active in FFvTTR as a 2 damage minimum is imposed.
The damage over time (DoT) is calculated as the power Magnitude/5. This is unaffected by damage multipliers, strength (for melee) or even by overpowering! The only factors that influence DoT are the power's magnitude, the upgrade level, and fluctuation (from 0.5x your upgraded magnitude up to 1.5x minus 1). Repeatedly applying these effects will add to the current DoT, up to the ingame maximum controlled by RPG_IRRADIATED_MAX=5 (this specific variable is shared with acid burn). Applying more than 25 magnitude (after fluctuation) of these effects is therefore always wasted, and 17.3 base magnitude is enough to roll 5 for the initial DoT.
DoT can be fractional, and is re-calculated every 1sec until it hits zero by subtracting RPG_ACID_DECAY_RATE=0.20000000298
So how foes this translate to the damage bubbles we see? The damage bubbles are equal to FLOOR(DoT) but are only dealt every RPG_ACID_DAMAGE_PERIOD=1.5 seconds.
Here's an example of the damage progress for a max starting DoT of 5:
(https://i.imgur.com/te66FXH.jpeg)
Strategy implications include controlled upgrading, and preferentially underpowering all hero acid burn & irradiate effects and using the C key to keep adding up DoT. (Sea urchin throwing 4 bubble swarms is very potent) When hero-building, consider increasing strength to improve success chance for melee effects.
RoR, that's very interesting! Fascinating work!