Don't want to tell a bloke to suck eggs so if you have done the following pls excuse...

Forget your first interpretations and have another close read of the cal procedure again just in case a comma or a word was missed.
Don't skip things like "menu exit" as often doing that stores a value and moving on to the next parameter means it doesn't.
Changing Tank size won't affect instant or average fuel figures, tank size only impacts DTE.

Out of the box..these OBDII units work by getting the injector timing, distance, etc, from the ECU and 'calculating' a ball park figure.
The distance you adjust with a known reference like your GPS so you have done that.
The initial tank gives you a fuel used number which then lets you adjust the figures to suit your vehicle.
Mine had a chip so it used more fuel than the ECU/Scangauge initially thinks.

Sooo... to display correct LPH then the gauge needs to know engine size (easy), calibrated distance/speed (GPS) and fuel type (Diesel obviously but which one is a guess) then after the first tank you will have a difference between what it thinks and actual.
Adjust the gauge for the actual amount and it should be very close and a few more tanks you can tweak it further if needs be.