Generally, I do not mix up powdered milk or instant milk either one for drinking in the BW, rather I mix it dry with what ever mix I am planning on using milk with, or that calls for milk in the mixture and then mix it all together at the same time when I cook it.  For desserts that I carry and prepare, I just figure the amount of powdered milk required to make the mixture and place all of it together in a small plastic bag which is rolled tight to remove air and placed in a larger stronger bag marked "desserts" or "Breakfast" to keep things nice and orderly.  By the way... the milk I use is from the USDA, or commonly called "commodity milk" and therefore no brand name... but really very good!!

Bruce