I calibrated my "FULL" mark by filling up the tank with fresh water. I removed the cover that held the Tank Watch tubes and had a friend tell me when the water level was at the top of the tank. I then re attached the cover, did the "Full" calibration as per the instructions and everything worked fine! Just returned from a 140 mile cruise with 4 adults aboard, (4 days) and the tank monitor read ¾ full. (3 green lights) This is a great tank monitor!