Yup... should say on your box/bag/bucket. I think most brands are 1/2 cup per gallon though. You might have to tweak it a bit to get the correct salinity you want. For example, Reef Crystals mixes up to 1.021 using 1/2 cup mix per gallon. For FOWLR
, that's probably OK. If you're going reef, you're probably looking for something closer to 1.025 which means you'll have to add a bit more mix per gallon.
Once you figure out what salinity you want, find a combination of nice round numbers to work with for cups of salt and gallons of water and it'll make your life much nicer in the end! Personally... I shoot for 1.024 and mix up 3.5 gallon batches, using 2 cups of salt mix for the 3.5 gallons of water.