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The sump will show the evaporation. The overflow will maintain a constant level. We keep our Auto Top Off sensor in the sump and it controls when to add water back to the sump.
Just make sure you know your running water line v. your system-down line. The running line needs to be low enough to accomodate all of the water that back siphons out of your tank when the power is shut down to the pump in the sump.