Tank full to tank full.

I.e fill the tank and run the car for while, keeping a track by the trip meter and then refill the tank.

Now you have the distance run and the fuel consumed.

Yes, tank full to tank full is the correct way of measuring the mileage.  I wish to add here that when we go for refueling (tank full), it actually doesn't get full (though it shows), as some amount of air remains in the tank.  So, in order to remove the air, the car needs to 'shaken' gradually a couple of times, and more fuel added between the 'shakes'. My experience is that it thereafter houses another litre or even more sometimes.  Besides, I have also experienced that the amount of air that remains in the tank in any case, is more when refueling is done after consuming less litres.  So, refueling (tank full) should be done after driving / consuming the fuel to the greater extent (say more than 20/25 litres).

For instance, after tank full, drive the car till 20/25 litres is consumed and thereafter go for refueling (tank full). This will help measure mileage more acurately.