ThanksThis is a little bit awkward for me because the U.S., for whatever reason, refuses to go metric. I'm also a literature professor, though the son of two math teachers. That said, I'll give it my best.
The tank volume measures, as a whole, 4.8 gallons (or 18 liters). So, if you're putting in 15 liters, I would guess you're coming close to filling the tank without quite filling it. Do you sit on the bike while fueling, so that it's level or fill it while it leans on the side stand? I don't like sitting on a bike while fueling, so I fill it while leaning, then hold it as level as I can while I add the last little bit of fuel.
The low fuel light coming on at 230 km (or 143 miles) makes sense to me. When I was doing predominately city riding, mine regularly came on at 150 miles. I could then get about 15 more miles (or 24 km) before it became imperative that I get gas (petrol, not the farts >). At that point, I usually put in 4 gallons (or, in liters, right about what you're putting in—15.15).
If I'm riding on the highway/interstate at a consistent speed, my low fuel light doesn't come on until ~190 miles (or 305 km), and I've been able to get 25 more miles (40 km) before filling up, at which point I added nearly the whole tankful of ~4.7 gallons.
Why your fuel light came on so early on the first tank is mystifying, though I seem to remember something similar with my first tank, actually. But that was many beers ago. It seems like now it's coming on at an expected level of fuel consumption. I'm betting your third, fourth, and hundredth tank will net you similar results.
I'm not sure where I read on this forum that people ate are getting about 300 km with city driving.