I looked careful during our 250m Cerb today and it looks like it is triggered when all 250m damage is done and the damage for 'last hit' mode is additional and not part of the 250m. That makes the formula a lot simpler. Since it takes as many as 3 rounds to get the last hit then you want to be able to do the full stated damage what there are at least 3 minutes left.

Required Health to be under at Given Time = Total Health * 55 / (Minutes Left -3)