You can do that. You do need to mix the entire gallon at once. It comes in metal cans, one can for each solution (which should also make it last indefinitely.) It's generally not good to mix partial amounts from powders, as the ingredients may not be evenly divided in the powder.
I just put mine into one gallon plastic jugs. It's not very subject to oxidation as developers go, but it's still good to squeeze the air out. What you propose is fine.
Well, it has nothing to do with Diafine per se, since no matter what developer is used you need fixer. And the liquid mixes down easily, lasts many rolls and it's just more pleasant if the smell isn't so bad.
Originally Posted by NedL
One nice thing about plain ole Kodak fixer powder is it works. One other fact is you are paying to ship from one side of the marble to the other, so powder fixer and developer (Diafine) might allow you more product per same shipping cost as doing liquids.
I am certainly blessed every time I smell fixer, it meand I am doing what I enjoy.
Start easy, learn a lot and love doing it, even if fixer does smell like fixer... btw, it is fixer.