The check valve you describe is either in the electric pump or the FPR (fuel pressure regulator) built into the fuel filter I forget which, but I'm leaning toward the pump since I get a lot of spray when disconnecting the supply hose.
This check valve does not seem equipped to hold pressure just to prevent back flow. I suspect part of the "oh shit the siphon foot is plugged, let the pressure out" system, in the case pressure builds in the siphon feet, the pressure would prevent flow backwards and let the pressure relief valve dump the excess fuel directly info right tank.
I don't seem to have any pics of the sender with the arm to measure the angle but I have discovered that the floats on both sides hit the top of the tank at a total of 82L of fuel (on my car at least).
55 and 27L.
The tops of both sides are symmetrical, so if they both stop climbing at the same time and at 55/27 the arm bend is correct (or the foot is properly engaged into the dent in the tank).
You should be able to put in about 10L once the test 6 numbers max out as the floats smack into the top of the tank. They should both stop climbing at the same time.
(You can also observe them coming off the top, that should be a lot easier come to think of it). At about 60-70 miles for me, I finally saw the numbers start to wiggle as I hit bumps or took curves.
If one of the sides starts to show a drop off the top sooner than the other i would investigate.
How the hell the computer guesses a DTE before the first 10L is consumed I haven't a clue. I think it assumes a full tank any time you are within 10L from the top; maybe explains why the DTE can swing pretty hard once the floats can actually measure how much fuel is in the tank.