For accurate fuel mileage you don't need to drain till the low fuel indicator. Even if you are at half tank it can be easily calculated.
However you need to calculate it from top up to top up.
Step 1: Top up the Tank
Step 2: Set meter reading to 0
Step 3: Drive around whatever Kms you want
Step 4: Top Up the tank again
Step 5: Note down the amount of fuel filled on this 2nd top up
Step 6: Divide the distance traveled by the amount of fuel filled and the answer you get is the km/l (mileage of your car)
Please be advised, I am not preaching or correcting anyone's methodology. It's just that a lot of people on Pakwheels talk about how they have to go till the fuel indicator lights up and then they've to fill up. I thought it might help. I do not intend to offend anyone with this.
peace
P.S: I get 9 km/l with a 97 civic automatic in city.