I totally agree a 600 pt disintegrate would one shot my 550 HP First Life numbers (anything above 550 would). However, thats all 3s on 100d3+300 and is worst case (Which is good to prepare for). During my time as Tank (again limited as far as Horroth is concerned) I saw the average around 450 (failed saves) even lower average if you put in the numbers from Saves. Still not alot of breathing room and I contribute my survival to three things 1) My understanding of how a Paladin's skills and abilities work 2) The high AC to help midigate the incoming melee damage and 3) the nimble fingers of the healer(s). All of which I see as equally important. 2/3rds of which place a heavy burden on me as the tank.
Again I'm not advocating Low HP. I'm only pointing out that it has and can be done.