I don't know about Mangrove Jacks specificly but generally 1 cup of salt per 5 gallons makes brackish water. Specific Gravity ranges from 1.010-1.015 anywhere in that range is acceptable for brackish water fish. A hydrometer will help with the SG reading. Instant Ocean or similar product wil be needed. You can't use aquarium salt it has to be salt for a saltwater tank.
Instant Ocean is the most well known by far, they make two major kinds..the purple reef salt, which is basically all you will need... besides for the hydrometer or even a refractometer that aqua mentioned. The reef crystals, or orange bag/box is more aimed at reef and full on saltwater tanks, so you won't need that...
DO NOT USE AQUARIUM SALT. That is for freshwater and is pointless.