Tractor drivers check the fill level of a liquid ballasted tyre using the valve stem position instead of measuring volume directly. The tube is mounted with the valve stem turned to the top, at the 12 o'clock spot, and water is poured in until it just starts to spill out once the stem is turned down to the side, the 3 o'clock spot.
Picture the tube as a circle. When water is added until its level reaches the horizontal line running through the centre, that is the 3 o'clock mark, the water occupies about three fourths of the circle's area, and only the top quarter is left as air.
So the 3 o'clock check that tractor manuals describe is really just a quick field way of hitting the 75% fill mark.
\[ \text{Water fill level} = 75\% \]