I have seen a few threads lying around about ‘how much fuel do i have left when the light comes on?’ and comments such as ‘even when its on zero, it just keeps driving!’.
well i found a cool parameter when messing around with my ecu dashboard the other day - ‘Fuel Level’ (%).
What is really cool about this, is that its a Live Feed. meaning its going up and down rapidly on every corner, braking, etc. whereas the needle you get on the Dash is more of a steady-Eddie which slowly drops.
Why this is useful? well, now i know that the fuel sensor is at the back of the fuel tank (fuel goes up when accelerating, rapidly drops when braking. no significant bias turning left or right)
I also now know roughly the landmarks for the Dash lights and Needles positions.
In short - the Dash is reading approx 10% lower at every point, compared to the ‘actual’ level of fuel the ECU is seeing in the tank.
Full Tank: 100% full at the garage. Dash says 100%, ECU reads 91% (its max).
The ECU will continue to see 91% for quite some time, until fuel level drops enough to reach ‘real’ 91%. (i.e, for the first dozen miles you will always see a fixed 91% even on hard driving and sloshing)
Initial Move: when the dash needle finally shows you have used some fuel, is when the ECU’s average is no longer higher than 91%. (roughly)
Half Tank: When the Dash shows 50% (needle at true bottom), the ECU is showing approx 60%
Low Fuel Light - this came on while i was driving so couldnt get a true fuel level due to motion, but its rough average at the time was 18/19% with an eyeball. the needle was just under the quarter. between 20-23% at a guide.
Zero Fuel: this then dropped very quickly in relative terms to the low Fuel Light on the Dash, because when the Dash showed the needle at Zero, i pulled over and let the fuel settle and it was showing 12% left in the tank. (approx 5…5 litres).
Thats a 6% drop in fuel, whereas the needle actually indicates about a 20% drop between Low Fuel and Zero.
Filling the tank: I was a very short drive to the garage forecourt for this test, ECU was still showing 12% remaining once it settled down. A full Tank was then an input of 37.5 litres for the first click.
Hope this data is of interest 
below - the current Dashboard im using, showing the fuel level at the bottom. its a shame its a screenshot, as during driving its a very ‘live’ number as i say.
also to note, for anyone wanting to really stretch that fuel distance - when the Dash says Zero, you may have 12%, and you may be able to get another 40+ miles out of it (as its over a gallon). BUT, as the live feed was showing me, the slosh on braking or cornering could very easily get you sucking up air though the fuel pipe. so if you are this low on fuel, just limp it to a garage 