mixing saltwater is annoying... its either too high or too low
Find a system that works easy for you. I use to have a 13 gallon trashcan with a line that marked 10 gallon. 10 gallons of water and 5.5 cups of salt. Consistent and easy. Get a couple 5 gallon water jugs if you only wanna change a few gallons at a time and store what is left. Mixing 10 gallons of water meant enough for weekly water changes for over 2 months on a 14 gallon cube.
There is always an easier way to do things and in this hobby, it's best to find the easiest way.