When filling up a motorcycle tank, since you can't use the pump automatic shut-off as a guide, it is a bit tricky to make sure you fill it to the same "full" every time. What I do is...
...turn my front wheel the same direction every time and stop at the pump so the bike is leaning towards the pump when resting on the side kickstand. I then fill the tank until the gas is just barely touching the bottom of the neck of the tank. Even a little difference can make a huge difference in mileage due to the small size of the tank.
While I don't have a street bike right now when I did I would fuel from on the bike. I would straddle the seat with the bike vertical and reach over to the pump to remove the nozzle, select the grade, and pay at the pump. That way I've taken as much variable out of the fueling process as possible. When you lean the bike over on the kickstand the forks and rear shock may not extend the same amount every time which adds a possible variable factor. Also, with a relatively small motorcycle tank by leaning the bike to the side you are giving up some of your limited fuel capacity. Good luck and do what works best for you, if that is leaning the bike on the kickstand, then great, or if you want to try to fuel it in the verticle position and maybe get yourself a little more fuel into the tank and a more exact fuel fill that will take some of the difference out of your mileage calculations.
concentrate more on riding and less on numbers... ;-)
over time the differences in the amount of fuel you put in the tank will be accounted for and be reflected more accurately in the average mileage... as long as you're pretty much filling up the same way each time...
I also fill to the bottom of the filler neck with the bike on the sidestand. Since my fuel tank is under the seat, I can't fill while sitting on it. In my opinion it's safer not to sit on the bike while filling. I just imagine the fuel squirting out into your face (unlikely but possible) while you are peeping in to see how full it is. The bike is dropped, spilling the fuel, etc.
I put my bike on its center stand to fill it, and fill it until the gas just touches the filler neck.
You can splash gas in your face whether you're sitting on the bike or not (DAMHIK) but if you're on the bike while filling up, there is a much higher risk of spilling gas all over your crotch. A friend of mine did that and then came to a party reeking of gas... it was so awful I asked the host to please lend him a pair of pants.
I agree with zanniek. On the center stand, fill to the bottom of the filler neck. I find on some nozzles, even as I pull back on the collar of the handle, it will shut off when it is full. I try to keep the tip of the nozzle at the base of the filler neck. Seems to work most of the time.