It depends, but about 2 to 3 inches is somewhere near. The far more important measurement is the one between the top of the hose -----once it has been turned down------and the bottom of the knee cap. These days, particularly with shooting hose( usually worn with plus 2's & 4's) being used with the kilt, there is a tendency for them to be worn too high on the leg for best effect with the kilt.
" Rules are for the guidance of wise men and the adherence of idle minds and minor tyrants". Field Marshal Lord Slim.