I think I have your answer. I too had the same problem. I have a 2012 f800st going on 5 weeks. I have 2,500 miles already so I have filled up quite a few times. Every time my fuel light went on I would only be able to get about 2.5 gallons or so of gasoline in the tank. I figured that the fuel strip was faulty and it was reading early. It did not seem like a big deal so I thought I would mention it to my dealer at the 6k service. Well I thought wrong. On my way to work last week it clunked out of gas just showing at 50 miles from when the light came on. A previous poster is correct you have to pull the fuel nozzle out a few times and carefully add small bits of petrol at a time to actually achieve a full tank. The fuel filling neck is a lot shorter on a motorcycle then a car so that is why you get a false fuel fill on the f800st.
I picked up my 2012 R about 2 weeks ago, the most I have been able to get in the tank was 3.694 gallons, that was at my first fill up. The couple times after that it was less than 3.5. The bike is currently in the shop getting its run in check.
On another note, my fuel gauge reads full until there are about 120 miles on the clock, after it finally disappears it goes into the basement pretty quick and the light comes on around 160 miles. I am guessing thats due to the strange shape of the tank.
Glad I rejoined MOA, I was hear a couple years ago when I had a F650GS. Looking forward to hanging out!
So 50 miles on reserve? That sounds reasonable.
Enjoy your ride!