Ian, with my bike on 4700 km now, and having done a tar trip from Great Brak to Bloem and from there to Kloof, my average on a tank is 360km. The first day, had heavy winds from 07:00 till 17:00. Average speed was a wobly(cause of the wind and not wanting to leave the missus in the bakkie too far behind) 115km/h for the 859 km stretch.
From Bloem to Kloof(620km) I averaged 120km/h.Less wind, slightly higher speed, still gave me 360 on a tank.
I must mention that I also had n set of soft luggage, weighing about 22 kg on the bike. The bags definetaly had a strong wind resistance effect.
PS: I noted, 1 up, very little wind, at 120km/h, I reckon I could easily do 380 on a tank.Little or no luggage.
Hope it helps.