So I'm not the only one! I've never got more than 20litres in it even after 50km with the light on.
I have found you can get about another litre in the tank once it is "full" if you really brim it ;-)
When I pulled the tank this morning there was about another litre in it, not sure how much the pump would be able to get of it.
Try it at your own risk, but my calculations based on my bike at 8ltrs/100km say over 270km should be possible for 22litres. My light normally comes on at 215km add to that the 70km I managed above gives 285 ...