Don't overthink, follow advice of Omega Man
I have a 2012 GS, fill it on the sidestand because I almost lost it a few times when loaded-up & on the centre stand. When I get to about 8 miles showing on the gauge, I reckon I still have around 25 miles left in the tank. I also top up as much as I can. Fill consistently, take note, and you'll not be stranded… most of the time.
Just my opinion as I come riding motorcycles with no gas gauge- I only trust it as an estimate. I would ride the bike until It took around 4 gallons to fill, make a mental note that when you hit that point on your trip meter, it's time to look for gas.
OM
I have a 2012 GS, fill it on the sidestand because I almost lost it a few times when loaded-up & on the centre stand. When I get to about 8 miles showing on the gauge, I reckon I still have around 25 miles left in the tank. I also top up as much as I can. Fill consistently, take note, and you'll not be stranded… most of the time.