Not filling up completely and to the same level every time will give slightly different results.
If you added 18L before the gas light thingy came on, you don't know how much gas was still in the tank. Even if the gas light thingy came on, it doesn't always come on at the same gas volume. It is just a simple float that is sensitive to acceleration, turns and ground level.
So unless you have a calibrated 18L tank you can use, that is probably the worst method.
A better method is to fill up the tank til it clicks off. Then drive as usual and then fill up again at the same pump etc and then calculate. It will be closer to actual but still lots of variables to throw you off.
Your mileage computer is probably most accurate for day to day use.
Other things such as traffic, temperature, weather, road conditions(hills, turns etc) your driving style will cause the mileage to vary. Without laboratory conditions you may never get the real mileage and even then it will change every time you drive in the real world.
One thing I do is keep a log book and chart the average mileage per page. Over many months you will start to see a running average that is pretty close. When the running average changes significantly, then something is wrong(tuneup?) or your drive changed.
Good Luck,
Jim Henderson