Does anyone know the total fuel tank capacity for a 1997 +8 please?

When I fill mine up the fuel gauge shows just over 3/4 full mark which I am told is normal as the VDO gauge never shows the needle at the full mark. So I am trying to calibrate how much fuel I have left at 1/2, 1/4 etc as I understand that when the needle is showing anywhere near empty then I'm running on fumes!

1997 +8 BRG, BMW Z4, Harley FXD, Greeves Silverstone RDS, Kawasaki H2 & Z1000 - no electric cars!