if you are using the fuel gauge to say you have half a tank, you will have LESS than half a tank. and the same with 1/4.
In my experience a tank only needs to be be 80-90% full to show as full on the gauge depending on the car (VAG will be nearer 80%)
in my mk 1 about 20% of the fuel needed to be used before the gauge would drop at all from 'full'.
in your example, if the tank is indeed 50 litres, and you have half a tank, that would be 25 litres, (5.5 gallons)
If you are achieving 44 mpg and have 5.5 gallons, you would get 242 miles to half a tank, not 200. This suggests that when the guage indicates half its actually leaving 20.7 litres, or 41.4% full. which is inline with my general experience that only 80% or so is needed to indicate a full tank.
stating how many miles you get per £20, or half a tank will not give you a real measurable mpg figures; you need to know miles and litres (or gallons)
so when the light comes on and you fill to half way how many litres are you putting in? i'd expect 20-21. so you clearly wont get half a tank worth of miles.
Based on 50 litre tank, (normally capacity is from full, down to when the light comes on) and 44mpg you are getting, this is the equivalent of 484 per tank.
does this sound better?