I am playing with some of my old cameras, just for the sheer fun of it. I expect to use up at most, a half a dozen rolls of 120 film. I have ordered the D-76, and Fixer in powdered form. According to the usage charts I would need only half the amount of material for the number of rolls. So the question is, can I just mix half the amount of powder and water and seal up and save the rest for a later time?