Trying to understand battery power descriction OLED screen

There is a battery symbol to indicate battery life remaining on one of the OLED screens inside the Meshtastic firmware. On the following screen there is battery voltage and percent charged.

How do I know in precise terms how much battery life is remaining? Battery “bars” in the battery symbol isn’t very precise.

I have fully charged my 18650 battery and used it in my device and drained it to the point where I can still see 1 battery bar on the battery symbol, but the battery charge percent is “0%”. Anyone able to explain to discrepancy. Much appreciated it.