Best thing to do is get yourself a hydrometer. takes the guessing out of mixing salt. and you can be sure that it will be the same salinity each time you do a water change.
I just take frequent measurements as I go along. You could get a small scale and go about it that way and get close, but I would never advise you to base it off of volume.
Volume is bad because the salt compacted more then the last batch, it can have a greater moisture content, or you could have voids within the measuring device itself.