It's hard to determine the level just by the bars displayed since a bar represents a range. Knowing what the level is when the number of bars change ( or when one bar STARTS to flash) can help but who looks at the gauge all day?. In addition, the angle of the hull affects the reading greatly in some cases. On my 208, one bar when it has just changed form 2 bars, can mean I still have 30 gallons of a 82 gal tank.
Getting on plane with an extra passenger, that one bar sometimes may increase to 4 bars.
I play it safe. If at rest at the dock, I have only one bar, I gas up.
I can't tell you how much gas it takes to fill based on the number of bars displayed. In 13 years of operating this boat, I can't remember ever filling the tank till the nozzle tripped