Note the meter reading. Let us call it KM1.
Fill the tank.
Use the bike till reserve hits.
Note the reading again. Let us call it KM2.
Now fill the tank. This time note the total litres of petrol poured into your bike. Let us call it LT1.
Subtract the KM1 from KM2. It will tell you how many kilometers the bike have been driven.
Divide this value on LT1 and it will give you per litre kilometers mileage of your vehicle.
Repeat this complete cycle two or three time to get an average.