Had a recent event on an A300-B4.
Tank 2 fuel gauge was giving + - 2000lbs in flight so at next stop we decided to use the mag sticks.
This still gave discrepancy between known uplift in litres, stick figure and gauges, so we put on another 2000lbs for mum.
Next stop we tried it from empty tank with a known amount from bowser and found sticks still gave errors of up to 1000lbs. (This was using charts for Attitude)
So I think the only time you know how much you have is when it's empty or full and overflowing.
Incidentally, with these errors, when we land at Max Landing Weight, which is quite often, has the manufacture taken account of the likely overweight case with gauge errors.
Actually with a MLW of 295,410lbs, I don't suppose 1000lbs or so is enough to worry about in line operations.