I have one 60 gallon tank for nsw and 30g for Rodi. I put float switches on the 60g so it notifies.me when it's empty and then when it's full. From empty to full is exactly one bag of instant ocean salt. I do awc so I get notification when it's low, then I dump a bag of salt in and turn awc back on. Been like this for a few years. Best decision ever was to figure out and plan on needing just one bag of salt. I never measure salt. I rarely test nsw for salinity. Btw, the 60g tank has served my system well. 175g display, 60g frag, 30g fuge, 80 sump. It's too small for a big water change. But big water changes are not a good thing.