kat is right the replacement water should be the same salinity and temp as the tank water. I tend to mix my water 48 hours in advance. then if I didn't add enuf salt, or added too much, I can correct it, and still have a full 24 hours of mixing and stabilizing.
Since you top off for evaporation with pure water (preferably distilled or R/O) rather than sea water, your tank salinity *should* stay constant.
Former advisor and planted tank geek...life's moved on though.