Turn your pump off. Then begin filling your display tank. Your water should overflow from the display to your sump. Continue filling until the sump is filled to the highest you would ever want to see, worst case scenario, ie if you had a power outage and the pump stopped running. Turn your pump back on and mark your level in that section. Now any evaporation will show as low water and you can manually top off to that line.
Next water change repeat and remark.