I supplemented with a Victron BMV-700 Battery Monitor, $ 104,
and can see exactly how much charge and how much current is flowing in and out at all times.
The thresholds vary slightly from panel to panel but they indicate based on the voltage seen at the monitor panel...
The % values are only accurate with little or no load on the batteries without charging taking place. If there is a large load on the batteries like from the inverter the voltages shown below will not be an accurate indication of the stored charge remaining. If batteries are being charged, you will see the top LED no matter how little stored charge remains in the batteries.
Top LED at >= 12.5 to 12.6 V
2/3 at >=12.2 V to 12.3 V
1/3 at >= 11.7 to 11.8 V
Battery almost totally discharged is <= 10.5 Volts
However, I suspect that the bottom red LED may light even at voltages less than 10.5
The main value of the bottom LED is to clue you in that you have operated the battery disconnect switch when you are not on generator or shore power.