A fair point, and I have had to operate single pack a few times. Practically speaking, though, if you lose the other pack, it means that you are going to conduct an emergency descent and then fly at FL100/MORA while diverting to a suitable field (you could continue to destination if it isn't far, but fuel consumption and the possibility of barotrauma injuries and pax panic are going to force a diversion in most cases). So the flight time is likely to be relatively short and the FL fixed. The only thing you can really do if needed is slow down to open a window with the outflow valve open to get enough through flow. I think trying to pre-calculate temperatures is over-analysing things.