it depends on the salt mix you are using. most will say on the side of the bag/bucket the ratio to be used. I just use a household drinking glass with lines on it and i know that i need 2 glasses for every 5 gallons as a rough estimate. Usually past then i only need to add a bit more salt to get it perfect. You will get much better after mixing up a couple hundred gallons and it will go much faster. Having a refractometer is also a huge advantage.