With the regular Series tanks you can adjust the reading by bending the float lever in the tank to correct the error. (However the first thing to check is that the float is not partially full of fuel.)
Not 100% sure on the 101, but you can also adjust the pick up tube by bending it down to just above the bottom (you always want to leave a little of space for the crud and grime to accumulate). If the pick-up doesn't have a bend, you need to lengthen the tube by brazing on an extension (in the middle so you leave the gauze mesh intact.
I did make the error, or at least have an unfortunate incident in the FFR, I didn't know how accurate the guage was, so with a full jerry can on the rear I decided to do the endurance thing on the way back from Corowa. At Phesants Nest there was about 1/4 tank and the next station on my run was at Arncliffe after the M5 East tunnel, at King Georges Road the guage showed the needle just hitting the zero line, so I chickened out running the tunnel on empty and exited. Good thing too as the fuel ran out in the middle of the King Georges Road intersection. Yesterday's was unfortunate because the vehicle had just had the coil replaced and the carby had been re-jetted, so I assumed it must have been a fault in one of those items. But as usual, I should have gone back to first principles, fuel, spark etc.