Assuming that you will have an 7-8 litre reserve that is pretty impressive. By my calculations 77-80mpg.
Apologies was using us gallons
1 litre = us gallon = 0.264172
1 litre = uk gallon 0.219969
42-7=35, 35*0.219969=7.69892, 600/7.69892= 77.933 mpg
42-8=34, 34*0.219969= 7.47895
I normally manage to get mid 600s from a tank , which is working out to around 70-75 mpg , however , the car has the myp 6 speed gear box, plus a range of the 1.2 TDI drag reducing mods , the winglets , Exhaust cover , and air dam , along with a re-map and EGR delete
It’s normally just me in the car , with the cc set to 58 mpg hiding behind a large hgv .....