You wont ever get the full listed capacity in or out of the tank
Kind of like the specs on cubic ft of storage capacity on an SUV, the rounded corners and crannies that techincally make up the total capacity cannot be utilized.
On the Spyder tanks, head space to allow for fuel expansion, and the space below the fuel pump inlet are part of the calculated capacity (listed at 6.9 gals for my 15 RT, 7.1 gal on wifes F3T) will never be considered on our trip plans for fuel stops.
Just got back from Lamonsters Spyders in the Ozarks, and had some head wind and cross winds that limited our average fuel economy to around 37 mpg running mostly at 66 mph.
We fueled up usually before 200 miles on the odometer, and coming home had one fuel-up after getting off I-44 in Galena, MO when my fuel light was on (but still on the last bar).
I fueled-up with 5.2 gals, and the wife took 5.156 on her F3T. We still had over 1.5 gal "capacity" but probably only a gallon of usuable fuel ---but lots of miles of smyles!!