More to the point you will need a hydrometer or refractometer before you add any saltwater to your tank. These are means with which to measure the content of your saltwater. doesn't matter how many cups, but that you have the right solution when you are done.
1.021 to 1.025 is the range of SG.
Since it is wise to let your saltwater mix for 24 to 48 hours you will have time to measure your salt in cups before you are done mixing so you will have to just flavor to taste so to speak, until you hit the right target range of SG.
Thomas