Right I get that....but you could choose to divide the time various ways. Sounds like the math is a bit loose depending on where you decide that cutoff is.
Examining dps "on the tanks" is pointless, it has to be the whole fight.
But you highlighted that phrase so I said how you'd see it.
Whatever you do someone still has to dps the adds and if you slack off there you have a knock-on with dps on the tanks so dps on the adds also has to be taken into account.