I don't calculate the odds on this, and I won't even try. I dropped the two healers in a 19th-level party on Wednesday, and the party still had resources to not only survive but win the fight. I nearly TPKed a 6th-level party (dropped the tank) this past Saturday, but the one character did a thing to pull their bacon from the fire. In either case, the party was clearly (to me) teetering on the brink of a TPK; both groups, I think, know how close they came--and that I wouldn't have arbitrarily or capriciously saved them by fiat. That's good enough for me.