Hi X,

From what you give in your post I could not make a skirt that would be guaranteed to fit the way the lady expects.

I would strongly suggest you have the lady take some actual measurements. This would include the length of the Fell.

Let her measure where she wants the waist to fit and where she wants the hem.

Please remember that the top straps of a girls skirt, worn lower than the natural waist, may need to be closer to the top of the kilt than is normal for a man's full rise kilt.

The thing about the Billie type skirts is that there is no Fell to speak of. These types of skirts use a placket to create the taper. Much along the idea of the "V" shaped placket in the back of a pair of blue jeans.
The pleats are sewn into the placket with little or no tapering.

Also, the swish factor is minimal when made with the heavier fabrics of a man's kilt. Most Billie type skirts are made from fabrics 10oz or less. 4 and 5 oz is not uncommon.