My goal when refuelling is
Use as much of the tank's capacity as practical.
A friend of mine never lets her fuel go below the half-way point, whereas I prefer to brim then use 97% of the tank. Particularly with the 126, where range is 200 miles max!
Always try to refuel from the same fuel level
Since I track my MPG, the key is to refuel from the same level of fuel each time, ie the same physical volume of liquid in the tank.
On my previous (modern) car, when the fuel light came on, the dashboard display would show how many miles were left in the tank. I'd wait until this figure turned to 15 miles and then I'd go refuel to the top.
Since the 126 uses a primitive float to trigger the light, I've taken to waiting until the fuel light remains constantly lit and then every so often I do a quick and short hard-brake: if the fuel light goes out, I keep driving. However if the fuel light stays on even when hard-braking, then I refuel.