How does honda decide on the capacity of their tanks? how far up the neck is considered full?

Has anyone completely drained their tank and then refilled it to see where the capacity gets to?

 

I usually fill my tank up right to the metal bracket where the gas nozzle fits in. Don't know have much more it could hold but I'm guessing not much. Remember the gas cap still has to close.

What I'd like to know is how much is left in the tank when only one bar is showing? or when it starts blinking?