hmmmm...

Well, I think the conventional wisdom is to just mix them as you go. Try that and see what happens, not too hard to mix a gallon at a time.
Get really fancy and get some 5 gallon buckets, make yourself a container with a spickit, attach some tubing, badda bing badda boom you got lil hose attachment. Mix them up to 5 gallons at a time if you wanna, but I don't know about letting them sit, I have never mixed more than I needed at one time.