I use reef crystals and a half cup per gallon is almost spot on. It at least gives you some frame of reference to go by when you are manufacturing the water. I have a container that is 5 gallons to the point where I fill. So 2 1/2 cups and a little more gets me to where I need. That way I really only have to test with my refractometer twice. Once you get a feel for your containers and how the salt combines you should be able to do the same.