There are some small holes at the bottom of the fork stanchions which the oil has to pass through as the forks go up and down. The rate at which the forks travel (i.e the damping effect) is controlled by the size of the holes and the thickness of the oil passing through them. Thin oil will pass through the holes easier and quicker than thick oil. So, to control the speed of fork travel you need to alter the thickness of the oil. Increasing the quantity of oil reduces the air capacity of the forks - If you fill the forks completely they would be solid! The quantities quoted by manufacturers take into account the air capacity of the forks, the spring weight and the travel. Unfortunately pattern parts are likely to vary in specification from original so setting up forks will, to an extent, be trial and error. The bushes need to be in good condition and the quantity used should be very close to manufacturer's spec.