Honestly i really feel like there is some misguided information here.
The poison stacks only have a certain time window to be added up, after that time period, the Dot stack restarts. There is no reason to do a weaker combo ever, under any circumstances unless you are going for a restand or HKD.
D3 and D4 or any stinger limb for that matter dont "increase combo damage" per say, what is happening is that the system is adding any exsisting dot ticks that occur mid combo to the combo counter albeit a very confusing system is in play. The timing is strange but go test this, and it will start becoming clear:
D3, 112 - if you do the 112 as fast as possible after the d3, you will only get 10 percent. if you pause for too long but still within the 5 second tick intervals you will still only get 10 percent. If you wait just the right amount of the time, the combo counter will display 11 percent after the 112.
Its also why combos that have more stinger normals in close proximity to each other tend to yield better results since the DOT damage is being applied within the small window before the stacking restarts. Go into the lab and do a basic:
212, d3, d3, d3, d3, d4xxdb4
Simply depending on your timing of the D3s, the dot will either tick for 3.82% or 4.16%
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A few things
meterless punish for raw damage
f44(212), d3, d3, d3, d3, d4xxdb4 = 40%(36%) corner only
best conversion off f43 for damage
f43, d3, f43, d3, d4xxdb4 = 32%
corner restand - dont waste meter for 2 percent damage
212, f33, f33xxdf1 = 33% plus restand