I apologize, I fired that answer off incompletely. For the "Track fuel-ups with trip miles/km or odometer reading?" setting, which way do you have it set? I prefer to track with trip distance, not odometer reading, and I didn't stop to think that the other option exists.
If you track with trip distance then there's nothing more you need to do. If you track by odometer reading, I'm not entirely sure.
Fuelly does not do a fuel calculation for the first fillup for this very reason. Enter a fillup, and 7 miles as the odometer reading. Fuelly will start calculating mileage with the second fillup.