My low fuel warning light came on today and I travelled another 20km before pumping.
There fuel pump auto cut off at 38 litres, which meant when fuel, warning indicator came on, there's at least 36 litres of petrol left (based on a very conservative 10km/l).
This means there's 9 litres of fuel left when low fuel warning light cam eon, allowing you to travel at least 90km before topping out.