Your mileage sounds about typical. There is an extra gallon (4.54) when fuel reserve light comes on. Allowing that you rode 13 miles before filling up with 14.76 liters, this equates to 3.24 liters left to your 18 liter capacity... so dead on...
So... you are getting approx. 33mpg (same as I do with mixed, but fast riding), hence you have 33miles left in tank when your fuel light comes on.