I was wanting to run some injector cleaner through mine a few months ago, so filed a 5litre can with diesel then ran the the A2 to the beep, zeroed the trip and then ran another 50 miles (the fuel needle was then on the empty side of the red), I then filled up the car and got 29 litres in (without burping it, but filled it fairly slowly)
So, I now new that fuel beep plus 50 miles = 29 litres, so I ran that tank to beep, then did a further 80 miles.
Removed the pipes from/to tank to engine, connected in slave pipes and a 1 litre jar with liqi molly in it and ran 1/2 of the liqi molly through the injectors / fuel pump. Then reconnected the tank feed and return pipes and put the remaining liqi molly into the tank. Then took the car for a spirited 20 mile drive. There was never any feeling of fuel starvation even on a twisty country road. Car back home, changed fuel filter and took it to Shell for a 33 litres of diesel.
My conclusion is that the 34 litre tank holds rather more than 34 litre of fuel even without burping it. I also now know that the fuel beep = 26 litre depending on how fast I fill it up, and that I can safely run it to 50 miles past the beep (typically if I can't find a shell fuel station or on motorway and fairly close to home)