ok... here we go. Maintained a steady 95 kph in Vic, to 105 kph in SA. No rapid acceleration, just gentle cruise. On downhill runs, pulled the clutch in, and idled along.
Had a minor milestone on the return trip, odometer clicked over to 170,000 km on the way home through the Adelaide hills.

Now, the logistics...

By google maps, point to point, 562 Km each way
By GPS, 558 Km each way
By odometer, 570 Km each way
Odometer end of trip reading 1,140.1 Km
Full tank leaving Portland at 8 am. First fuel stop at Greenacres in Adelaide, a few k short of destination at 2.15pm (SA time)

I did add 4 litres from my side pannier at Penola, then put in another 5.24 litres splash at Mt Gambier, to ensure enough to get home.

Filled tank at home, 19.36 litres

Total fuel used, 51.72 litres, for an odometer reading of 1140.1 Km ... 22.04 Km / litre... now that's economy!