I'd never trust an analogue fuel gauge.
From a full tank record the NM you do then see how much fuel is needed to brim the tank again. Over a few runs you'll get a working figure - L/NM o or NM/L you will be comfortable with, but keep a running calculation over time as on ongoing check
I'd also plan to keep a reserve of c. 25-30% in the tank at all times in case it's ever needed.
Happy ribbing