Well, that's the thing. You can't ration it, exactly, but you can calculate how much a person uses, on average.

Earthers use the most - they've got denser bones and muscles and so need more oxygen for their metabolism to keep up. And people use a lot more oxygen awake than asleep. So you figure out how much your crew needs, then make sure you have enough that with the recyclers you can get to your destination and back - with air to spare - even when considering the losses that naturally come with the recyclin' systems.

Water's the same way, though the losses on the reclyin' is a lot higher.

Hell, you're more'n welcome to it. Just let us know if you manage to get anythin' out of 'em.