It all depends on what you want to do, and how much your tank uses through calcification in corals. You can either saturate the solution (add more than 2 tsps per gallon) and even oversaturate it by adding vinigar. Any extra above the saturation level just settles to the bottom, and resaturates when you add fresh water. Then you can adjust by only replacing, say 50% of your reef's evaporation with it, and using an auto top off system as well. This is what I do. The other option is like Debaser said. Figure out how much less lime you need below the saturation to just keep up with your reef's demands.