Can I mix only part of a bag of D76?

I've used those vacuum wine stoppers for wine bottles on my bottles of dev - fit really well and have worked like a charm over the past 12mths.

My folks had a spare set that they got as a gift and happily gave it to me to use - they only need one pump and have several stoppers, so the extra one sat around for ages.

http://www.amazon.com/Vacu-Vin-3-Piece-Saver-Stopper/dp/B00004SAF4

I don't understand why you would want to mix partial bags of D76, when I can get a litre bag for the same price of a portion of chips. I did wonder though if you want to mix up lower measures of a dev or whatever would it not be worth buying raw chemicals and a set of scales that measure in the fraction of a gram? Obviously it may work out more expensive to buy the chemicals this way. I do worry about loss of activity in my devs though and have taken to dropping a bit of the films leader in a few drops of the dev to make sure it is still working.