Photographers Formulary and a scale is most cheap. No penalty for mixing a liter.

D76 or ID11 has a 6 month life after which it dies slowly. 50% full bottles are good for a day, no more. After that activity goes up rapidly, then dies fast. It is not predictable. I have done tests, very careful accurate tests.

Rapid fixers are good for 6 months, after which they sulfate out. You need this type for TMax and Delta films. For others you need plain old powder sodium thiosulphate types from powder or Rapid types also work.

For home developing you must decant what you make into small one time use bottles.

Beware of Sodium asbsorbate developers that do not turn brown as they go bad, Xtol, and the liquid Ilford developers. Actually you do not not how old they are when you buy them and depend on stores stock rotation. Bad developers=low contrast.

Do not even think of breaking down a gallon bag to make smaller ones. #1 the chem are not necessarily mixed !00% and #2, they are packaged in a nitrogen atmosphere to retard oxidation.