i know on the race car we run we have a tank in the boot and a calibrated wooden stick to see how much fuel is left in lol, maybe it would be easier for you

(only joking sorry i not much help)
my old capri fuel gauge never really worked so i always drove 100 miles filled her up till fuel drained out of the tank on the floor then i knew it was full,
on the e-type i tend to open the fuel cap and look in, if you can see fuel, there is enough to get you to the petrol station if your not sure. i dont trust the old smith gauges