ma sha Allah congrats brother for the car and also for the awesome fuel avg.
but your calculation as i understand is based on assumption of how much fuel has been consumed from full to half tank indication.
To get the most accurate fuel avg , follow these steps :
1. get tank full and ask him not to over fill it just leave it where it cuts off automatically.
2. reset your trip A meter to zero
3. drive around till like 20-10% tank is left
4. refilll again as in step 1 , and divide the total kms in trip A with petrol in litre that filled in 2nd fill . that will give you the most accurate fuel avg
saying all that the avg is almost never too far from the avg shown by the digital meter. like i get 21-22 km/l avg in city lahore. and the avg meter also shows between 21-23 .