Mixing salt is the most repeated job in the hobby and the easiest to do approximately. The cost of approximately is a water change that arrives at a different salinity than the tank it is going into, which is a swing you inflicted on purpose.
Worked from your tank's real water volume rather than the number on the box. The calculator itself needs JavaScript; everything it assumes is written out below.
Mix to the salinity the tank is at, not to the salinity you wish it was. A water change is not the tool for correcting salinity; topping off and evaporation are.
Weigh the salt rather than scooping it. Salt mix settles and compacts in the bucket, so a scoop from the top and a scoop from the bottom are not the same mass, and the difference shows up as inconsistency you will blame on something else.
Give it time and flow to dissolve fully before it goes near the tank. Undissolved mix keeps reacting after it is in there, and a cloudy bucket is a precipitation event you are about to pour into a display.
About 136 g per US gallon, from the 36 g per litre most mixes need to reach 35 ppt. That is the calculator's default, and the field beside it takes the figure printed on your own bucket, which is the one to use, because mixes differ by a few grams per litre and the difference shows up as a salinity error on every water change.
35 parts per thousand, which is a specific gravity of 1.0264 at the temperature reef refractometers are calibrated to. Some keepers run 34 to 36 ppt without trouble; what matters more than the exact number is that new water matches the tank, because a water change at the wrong salinity is a swing every coral in the tank feels at once.
Subtract 1 from the specific gravity and multiply by 1326, so 1.025 is about 33.1 ppt and 1.0264 is 35 ppt. The card on this page does the conversion in both directions. Hydrometers are calibrated to a temperature, and reading one at a different temperature is the most common source of a wrong salinity number.
The rules that apply to every dose rather than to this one, including never dosing calcium and alkalinity together, are on the <a href="/tools">calculators hub</a>.