Generally speaking, it's pretty accurate, as it is reading from the ECU. It's certainly better than an aftermarket thing like the Lowrance in-line sender type thing--I could never get any of those to better than 10% out, regardless of doing a calibration. It does rely, of course, on you remembering to manually enter any added fuel.
I actually drained the tank last year to check after putting about 800 hours on the motor, and it actually had 20 litres more in it than it should have. The fact it was exactly 20 litres made me think I'd put a jerry can in and not entered manually. Anyway, either way, pretty accurate.
My system won't read the fuel gauge either--it was never set up properly by the installing work shop--you need to be empty, then add as you go. Next time I empty , I'll organise to try it then.