the simple answer as to how i measure so accurately is one of my vehicles has a tray top mounted fuel tank that i fabricated , Its exactly 200 litres to the filler neck , I have used it for backup when doing site calibrations but conveniently i also use it for reserve fuel to fill my mini excavator or forklift which i can carry on the truck , i can also push a button on the dashboard and transfer fuel from it into the truck tank too.
Its very difficult to get an accurate gauge from the OEM fuel tank on vehicles these days because they can be such an irregular shape and have voids in them , plus vehicle tanks shouldnt allow for 100 % filling to capacity due to expansion gap anyway.