I just recently acquired a 2019 model CB400X (the rest of the world get a CB500X). I got this bike (I named her Friday as she seems up to all I have asked of her so far) because of the 17L fuel tank.
My experience with her so far has been averaging about 500km on each tank up till the flashing of the fuel gauge indicating reserve.
Other than her being a bit overweight (195kg is no sneeze for a bike putting out only 42bhp), she is reasonably torquey.
I believe that cruising at about 4500rpm should give a range of 450km (I'm guessing) comfortably for a 500cc powerplant..
My 2004 R1150RT get 35 MPG in the city and about 40+ on the highway. It has a tank right around 6 gallons which means that if I wanted to run it until the fuel light came on I could get about 240 miles out of a tank. I typically don't let it get that low though, unless I have a spare gas can in my saddle bags.