I check mine before I add top off water. I was told water will evaporate but the salt doesnt. So that being said when I do check I have found my salinity to be a little higher before I add to compensate for the evaporation. This by no means has any scientific proof and I am curious if any one else has noticed this. I does work in my situation.
Don't worry about it Miz if your topping off to the same water level you started with then your top off will bring the salinity back down to where it is supposed to be. If you add saltwater for topoff then you will see the salinity go up. Right now my 180 is going through about 4g of top off per day.
My concern is low salinity. On my refractor the #'s on the right (salinity right) read -30 where's the #'s on the left (SG right) read 1.025 so I am understanding this as my salinity being low. Another reason I made a thread asking how to read a refractor meter am I reading it wrong?? Lol