I don't have fraps to confirm this, but I am almost positive you are correct and that they spawn based on his life. At least what we did before we got geared is DPS the boss until the second set of adds came, then DPS only the adds until they all died. No more adds come during that time. Once all the adds are dead, and the healer has energy, we resumed DPSing the boss and kept on him until he died, ignoring the adds that spawn at that point.
I've straight up burned the boss down and only 2 packs of adds spawned. I've only seen the "full" 4 packs one time, when the group had absolutely terrible DPS. It's not based on life. If it were, the same number of packs would spawn every time.