It is my personal opinion that denim makes a terrible fabric for kilts. It is a very stiff and unforgiving material. It's ok for jeans because you are inside them, always stretching the fabric out to shape. In a kilt the fabric WILL shrink and pucker. Look at the seams of your jeans, see those puckers? That is what each and every pleat edge you stitch down will soon look like. Unless of course you just love ironing.

Not everyone agrees.

But if you have decided on denim then my comment would be to include as much fabric as you can possibly afford. My work weight kilts have between 7 and 9 inches of "Sett". (depending on model) With a reveal of either 1.5 or 1 inch that makes the pleat depth between 3.5 and 4 inches.

On the left side of the outer apron is the deep pleat. This pleat needs the depth to allow the aprons to hang correctly when standing and to allow room for you to sit and have the apron fall between your legs. On my kilts this pleat is half the depth of the width of the apron.

On the right side of the under apron is the reverse pleat. This is just like the deep pleat but because it also reverses the direction of the pleat it gets a special name. (Think inverted or reverse box pleat). On my kilts this pleat depth is also 1/2 the apron width.

I hope this helps. I'm not trying to discourage you from your endevour but trying to help you from making something which after two washings starts to look like a tutu.