do you have a cover on the tank? if not covers are best to use so that you con't get much water loss, also placing one on your sump helps too. your top off would have to be on the return section of your sump if thats your means of filtration. if not its just added clutter and a well fitted cover will work fine.
i had problems with evaporation, i would loss a gallon to a gallon and a half a week, once i made a full plexiglass cover with minimal gaps around my filters and wires i can get to my next PWC
with next to no water loss and a stable salinity.
as for as water changes you'll still have to do the same change, when water evaporates like the salt the polutants stay in the water makign the concentration higher in the remainign water