It would be good to know what sort of kilt you are making, whether contemporary or traditional and also what fabric wool or cotton, and also whether you are machine sewing or hand sewing.
Basically the taper is determined by the difference between the hip and waist measurement at the back of the kilt, a rough guide is that the back of the kilt is about half of the total hip measurement, you then do a calculation based on the number of pleats needed and then that number is divided into half the waist measurement to give the width of the pleat at the top( or nearly at the top).
There are so many variables it is hard to be more helpful, how about some measurements and I'm sure that many of us wil be happy to help.