Currently, the displayed DPS for Blight (the active skill gem) is the value for a single stack. However, unlike most chaos dots this skill actually applies a separate layered stack of the dot every time the channel ticks on a target (base cast time 0.3s), max 20 copies at once. These all expire independently, so for sustained channelling, on average there will be (debuff duration)/(cast time) stacks up at the same time.
So eg. in 3.8 with no modifiers there will be (2.5s duration) / (0.3 s/stack) = 8.33... stacks up at a time, effectively a +733% more multiplier on the listed damage. This means that bonuses to cast speed and debuff duration, multiple spell totems etc. all turn into extra damage, up until you hit the cap of 20 simultaneous stacks. In most cases with a relevant build you'll just be up against the stack limit all the time for +1900% more damage, assuming you can sit there and channel all day without having to move.
While this math can be done by hand if you're aware of the need for it, it's the sort of thing that would be nice to have just work automatically in PoB.
Thinking about it more, I don't know if it's possible but it would be nice to also have some indication of the ramp-up time and what percentage of your time needs to be spent channeling to maintain full stacks. A build that can spend 2s building stacks and then 8s running around is much better than one that has to spend 100% of their time channeling just to maintain 20 stacks, even if their full-stack DPS is the same.
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