The only way to know how to interpret the gauge is to ride until cutout, once off it won't harm the batteries greatly and atleast you will have a feel for the gauge.
The bike will cutoff power at a preset voltage that also tends to vary (like the gauges) a touch but should be above 80% discharge (one of my controllers cuts off at 50% it is annoying but great for battery life).
A digital gauge will be a little better but you would still have to run to cutoff to find the cutoff voltage.
Bare in mind as well that an increased load ie. up hill a heavy bag of groceries will affect the cutoff due to increased voltage sag, temperature and battery age will also affect sag and thus cutoff.
A reduced throttle setting will often get you home if you hit cutoff by lowering the amount of voltage sag due to load.