When doing topoffs, you only want to use plain water.
When doing water changes, if you keep up on your tops offs you just make the new water to the salinty you want.
After awhile, you will just get a feel for it.
There might be a simpler way but this is what I do now. I have a mark on my sump to show what the water level should be. As ong as it is at that level, then I know my salinity is good. I now have an auto top off so I don't worry about evaporation as much
*180 gal Display, 100 gal basement sump, 33 gal refugium, 3x250 MH, 2x160 VHO actinics, zoos, some softies, LPS & lots of acros and other SPS.
*100 gal prop tank plumbed into main system w/ 2x96 PC lights and 1x150 MH,