Its a half cup per gallon. The one baggie will make 5 gallons. You need to measure out the second baggie since you don't need another 5 gallons what with the rock and sand.
grab one of your moms measuring cups and use that til you can get to a dollar store and get your own. its only salt so its safe.
using 1/2 cup of IO salt will get you to approximately 1.025 salinity.