the waist measurement is divided into two ( very roughly at least ) to make the front and back of the kilt. The underapron is the same width as half of the waist. so in actual fact you will have 3 sections to a kilt, but the total length is about one and a half times the waist measurement.
It's a lot more complicated than that, which instructions are you using? if you tell us , I'm sure that some of us can help you through.
and good luck with making it.