I fitted a NASA on my sea boat 18 months ago and very recently fitted one on my sons widebeam, I have learnt from experience that the "state of charge" indicator is fairly accurate providing the batteries receive a full charge from the shore power 3 stage charger from time to time, if as has happened recently when the only charging available was via the engine alternator, the SOC indicator was fairly inacurate, however, because the NASA gives a very accurate voltage and current reading (very handy for seeing how much juice is being used by various appliances, even 240v stuff via the inverter) I can determine what the SOC is approximately enough to decide if charging is required.
Jim