By the way- take into consideration how wide the fabric is, too- not just how long it is when buying fabric.

For example, if the fabric is 45 inches wide from selvage to selvage (the finished sides that won't unravel), to get eight yards of fabric that are 22 1/2 inches long, you only need to buy four yards, cut it in half, and either hem the top, put a waitband over the raw edges, or hem the bottom. Also, add in plus whatever you might want for matching sporrans, belt loops, etc. (Waistbands are the tricky part to measure out when estimating yardage!) Just make sure that the seam where you join the two pieces together is the point of an inside pleat or somewhere non-noticable.

Iif you're going to need longer fabric than 1/2 the width of the fabric you'd have to buy as many yards as you want in the finished kilt.

I like knife pleats- which are a three to one ratio. So- I need six inches for each two inch pleat- Two inches outside, two inches back inside, two inches to the start of the next pleat. Try folding graph paper the way you want your pleats- it will give you a good idea of ratios, and a thought on how you want the pleats to hang.

Here's a good page on pleats in general, it's for sewing historical garments, but it's the only page I can find with most kinds of pleats pictures in one spot:
http://costume.dm.net/pleats/