The underseat tank is a funny shape and is a long way off full when the pump auto clicks off, especially if the diesel is foaming!
I'm sure I've managed to get >50 litres in a few times (I've had the car 10+years).
With lots of variables (tyre size, right foot, working gauge, working warning light, brimmed or not etc...) I worked on the principle of ~12gallons at 25mpg gives me a 300mile maximum range. So I try to fill up after 250-275 miles.
Only been caught out once (and that was in a different car) when I relied on the warning light and the bulb had blown