I thought about this earlier in my film processing habbit.
When I used powder type (I now use Ilford rapid which is liquid), I always mixed the whole bag. It'll give me a gallon. I use it for 6 months or exhausted by number, then replace it. Sometimes it's wasteful but a bag costs something like 6 dollars and I can have it ready for 6 months. It's the smallest part of the entire expense.
Paper costs much more. Gears cost much MUCH more.
Now that I use liquid, I mix a gallon worth and divide it into 1/2 gallon bottles for fix1 and fix2. Then the remaining concentrate is used 6 months later.
I don't worry too much about economy of chemicals anymore as in absolute terms, they are the smallest part of my expense.
If you want to mix-for-use type thing, you might want to use liquid. Making partial bag is an often discussed topic on apug and there's no solid answer. It's always a "mixed bag". If you shake it well and use half of it, it should be reasonably close. Yet, I don't practice this method.