I think the simplest answer is also the best in this case. This should work for (what I presume is) the majority of rationing cases.
Q: Why are you rationing?
A: Because you do not have enough food and need to survive until you
get more. Once you run out of food you will have no option to ration,
as then your only choice is to fast.
I think that the "do not have enough food" part is key here to make sure we are on the same page. If you have enough food but are not eating it, then you're dieting, not rationing. Maybe this is not correct given the technical definitions of the words, but I think this fits with most peoples' view of what it means to ration: to limit the use to avoid running out entirely.
If I have 3 days worth of food but I think I might not be able to get any more food for 6 weeks, then the math is simple. I can...
eat normally for 3 days (I'll assume 3 square meals per day), then hope that I can go completely without food for 5 weeks and 4 days more without dying, or
not eat for a few days (assuming I'm initially healthy, otherwise skip this initial fast), then eat half of what you consider a normal meal once every two days, then hope that you can go completely without food for a few days more without dying.
3 days * 2 times less food per meal * 2 days between eating * 3 times
fewer meals per day
= 36 days worth of rationed food instead of 3
Would you rather start a 39 day fast while starting in your normal, healthy state of body or a 3 to 6 day fast in a less healthy, starving state of body? You probably will die in the first scenario since you likely cannot survive 5 and a half weeks of zero nutrition. Yet many people all over the world are used to eating several times less food than you or I every day as a normal part of their lives for their entire life.
If you risk death from having zero food, we are told by the experts (I use that term loosely, but I would rather heed the advice than risk death) that you will die within a few weeks of eating nothing. A few weeks. People do not starve to death from a few weeks of severe malnourishment when they eat a tiny amount, rather, they live long, unhealthy lives for months or years.
You can stay alive for a long time in an unhealthy state of starvation as long as you keep eating small amounts of food, but once you stop eating altogether (which is what will happen when you run out of food from eating it to soon) the timer to a 100% certain death starts counting since people generally survive only a few weeks without eating anything no matter how much they bulked up first.
So the rationing question becomes this...
Choose one: Do you want certain death in a few weeks, or do you want to live a malnourished, unhealthy life for a lot longer and maybe (or maybe not) die in months or years?
If you are do not know how long you will need to ration, you need to make sure you do not over-ration. You can die from starvation even while you are still eating sparingly.
Some people have died from hunger or thirst even when they had more food or water available. They rationed so hard they died before using all their supplies. Please don't die of thirst when you still have water left. How much is the right amount? It's hard to say, and either way is a gamble.
If you are stuck in a dry mine, the middle of Antarctica without a heat source, or in a prison cell after a prison is abandoned, then I am sorry for you. Otherwise, food and water can be procured in the vast majority of locations on Earth, even harsh ones. Ration softly and spend your remaining days as a race to learn to extract food and water from wherever you are.
See also this other outdoors.SE question where the amount of time one can survive with zero food is discussed a bit more.