If you look at the shape of the tank and wher the pump sits in the well,,,, and then look at where the fuel gauge pick up is, you'll notice that the fule gauge only measures the slab block part of the tank, not the well.
The well would hold a considerable reserve, around 10 litres would be my guess.
I can usually get 500km before I have to fill up. Guyra to Gilgandra highway miles, 520km, I put 52.7 litres in. R33 Gtst 18psi and 290+rwkw, but you don't boost much unless overtaking. Who says you can't have [a bit of] performance and economy.