The boxes and buckets from the manufacturer Instant Ocean are labeled with how many gallons of saltwater it makes. Use a refractometer or hydrometer towards the end of filling.
This is setup-specific. As tom said you will need a quality refractometer and educate yourself on calibrating/using it--non-negotiable. For reef tanks you want to be sitting right around 1.026 sg. Ive heard you can keep fowlr setups at lower salinity but always tried to keep 1.026 sg when i kept fowlr. There are exceptions and caveats for specific corals, invertebrates, and fish that may have specific requirements.
I do know the main objective is stability in salinity, especially if youre keeping corals and other expensive, sensitive critters. Im no reefer but i do know anyone serious about keeping corals invariably has an auto-top off system installed on their setup to offset salt creep/evaporation.
I dont know about brackish tanks...obviously lower than a marine setup.
As a generic measurement number, to achieve a salinity of 3ppt (slightly brackish), @3 lbs of salt per 100 gallons
To mimic sea water (35 ppt) 35 lbs of sea salt per 100 gallons,
depends on what salinity you are looking for, sea water? That's at least 155 lbs of salt, in 440 gallons. But......
The Red Sea, can be slightly more saline.
Just buy the cheapest saltwater mix if you’re just keeping fish. No need to get expensive reef specific mixes that have additives for your corals since you won’t be keeping any.