I am pretty sure that the fuel gauge mark 1 refers to tanks max capacity NOT including the reserve quantity, which is approx 6 lt. Therefore using Litres as reference unit, mark 1 corresponds to 44 lt and mark 1/2 to 22 lt accordingly.
In the following picture quantities can be clearly seen in case 1 corresponds to 50 lt and 44 lt accordingly.
I base my thoughts on two different observations:
1)A had a trip of 486 km with average consumption of 7.3 lt/100 km, therefore consumption was 35.5 lt. Having a full tank when voyage started, ie 50 lt, the remaining fuel should be 14.5 lt. But the needle was below the first line and in case that mark 1 is 50 lt, this means that remaining fuel was less than 12.5 lt. Something must go wrong...
But what if mark 1 is 44 lt, then the needle was showing approx 8lt something that makes sense.
2)I visited the gas station directly once the reserve indication light appears. The needle was at the point indicated by the green arrow. Lets say mark 1 is 50 lt and green arrow 6 lt (when reserve light appears). I filled up the tank with 30.5 lt and then needle then moved to the point illustrated by the blue arrow. Therefore we assume 6+30.5= 36.5 lt, but as per picture above it should be approx 42-43 lt.
But what if mark 1 is 44 lt? Then the needle will indicate approx 37 lt, which is normal.
Furthermore and as per fellow 'Crocket', he was unable to fill up the tank with more than 45 lt from 0 indication.
In view of all above I understand that 0 indication corresponds to 6 lt and when warning light comes on does not mean that you enter reserve capacity but you should consider to refuel your vehicle.
Other fellows' opinions/experience are very welcome.