There are two places to take the reading: at the gauge by the tank, or reading at your monitor panel.
When the tank is filled correctly by a knowledgeable person you should see "full" on the monitor panel. The needle on the gauge by the tank may actually read somewhere above 3/4 full since the tank requires a 20% vapor space.
The monitor with readings of: empty, 1/4, 1/2, 3/4, and full are actually showing the reading when the tank liquid level is near the sensor mark. That means the "full" reading LED will go out rather soon since the tank should only be at 80% (liquid). Remember that you when you see "3/4 full" it means the tank is somewhere between 3/4 and full (just below the 75% level when first noticed). When 3/4 LED goes out and you see 1/2 full you are somewhere in between.
Thing to remember is that you would only know it is somewhere over 1/2 full. Unless someone checks often and happens to catch the 3/4 LED just go out you do not know you just went below 75% level. The needle-gauge at the tank tends to read lower.
I play it safe and tend to refill the tank soon after the monitor looses the 1/2 full LED. If propane is not available close to my location I make sure I refill when the tank gauge needle is dropping close to 1/4 reading.