I've been that route and find that just resealing the bag in the box doesn't work unless you use the dektol pretty fast. But then there would be no reason to just mix part of it. Sealing it in airtight containers is good, especially if you can displace most of the air out of the container while doing it. I'm thinking maybe freezer bags, squeeze them down before sealing that last inch and put them away in the original box. If the powder turns brown on you just toss it, you won't like what it does to your prints.