My plus 4 is similar; it generally shows just over 3/4 with a full tank.
An alternative is to let it run to showing virtually empty…locally; ( carry a spare can if you’re worried about getting stranded), then go to a filling station and fill the tank until the pump cuts off.
Under those circumstances on my Plus4, I can only get about 37 litres or so in, so with a 55 litre tank, I have confidence that I can get at least 100 miles before getting concerned, even allowing for some that may not be pumpable in the bottom of the tank.
