My take is to top up with 50% of your salinity... Meaning of your salinity is at 1.024, you top up with 1.012... This is because your water evaporates probably as fast as your skimmer pulls out... You top up evaporated water with zero salinity, right?
Therefore if your evaporation is 3 times faster than your skimmate, then you'll need to top up water of 1/4 of 1.024, which is 1.006...
This observation is only my 2 cents worth...
Angel Keeper