I had a "Quick tank" auxiliary cell on my '06 Gold Wing until I sold the bike. Whenever I stopped for food/fuel/"pit stop", the gas fill up was the last thing I would do before leaving the gas station and I filled the main tank to just beneath the fill tube and filled auxiliary tank to just beneath the vent outlet. Once on the road I opened the auxiliary tank value for about a minute of two to fill the line and bleed off pressure from the auxiliary tank, then shut off the valve. About 50-75 miles down the road I would open the valve for about 5 minutes to bleed off more pressure.
Depending on speed, wind direction, and altitude, I was good for close to 500 miles.
The only time I had fuel spill out of the vent line was when I completely filled the auxiliary tank and let the bike sit in the sun.
Of course, as always, YMMV. The '06 below...