There is a really simple solution to your question...and all it takes is for you to make 1 5 gallon batch.
Put your water in the bucket.
Add a few cups of salt and test the SG
Add more till the proper SG
Write down how much salt you used.
Now you know, when you use that same brand, it should always use the same amount of salt. FWIW, I never mix salt by volume only. I know about how much salt I need to reach 35ppt (which is the salinity of NSW
at most reefs around the world) but I check with a refractometer to be sure.