The stitching of the Fell ( the part from the top of the kilt down to theend of the stitching) is generally measured to be about one third of the full drop of the kilt, but a often this is adjusted to be "on" the point of the widest hip measurement.
The reason for this is that the pleats can swing free below that point, if the fell is stitched too low there won't be a good swing , and stitched to high the kilt will probably swing with more of a kink and may even crease more easily.
All these measurements are based on the judgement of the maker as well, and there is no absolutely "exact" number.