Overcharge starts at level 0 and is leveled to 1 for the first transition = 2 Overcharges
If you level Overcharge during P2, it will level again for the 2nd transition = 4 Overcharges
If you only level Diffusion chain and push the 2nd transition without leveling anything else = 3 Overcharges
Which is why we do P2 and get 3 Overcharges, one in each quadrant - if you level Overcharge during P2, it levels again for the transitions and means that you'll end up with 2 Overcharges in one quadrant for sure!
Either way, even if you aren't leveling Overcharge during P2, go 3/3/4 where one of the 3s is both tanks and a self healing class.
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^^ I could be wrong on that, but that's the impression I was under.