I mix powdered milk with my granola and put each day's portion in a plastic bag before leaving home. I put in more than would be called for to create a cup of milk for the granola. It seems to work well no matter how much you put in.

I also mix in freeze dried coffee with the milk/granola mix so I have my coffee and granola all in one. In cold weather I add hot water but usually just add cold water to the mix.