Typically it's the *hip* measurement, not the waist measurement, that really governs how much tartan you need. You want the pleats to look good at the hip, and most kilts will have some taper toward the waist. This is particularly true for people with smaller waists, where their hips are likely to be 6-8" bigger than their waists. If your waist were 36" and your hips were 42" (I've made kilts for many guys who have these measurements), you wouldn't have nearly enough tartan if you bought it based on the 36" measurement. So, its the hip measurement that is really critical to determining yardage needed. The caveat to that is if your waist is actually bigger than your hips - in that case, you'd work with the waist measurement, because, under those circumstances, your kiltmaker will make your kilt with no taper in the pleats and no flare in the apron.

And, I'd say for myself that I typically buy more than 8 yards for anyone with hips over 44" unless I know that the sett size is small. And that could easily be someone with a 42" or less waist....

Also, how much tartan you need will depend in part on the size of the sett. If the sett is fairly small, and you want the kilt pleated to the stripe (traditional, same stripe on each pleat, rather than alternating), it doesn't take as much yardage.

Anyway, your kiltmaker would be happy to order the tartan for you and get what you need without getting a lot extra. Having said that, PaulHenry is right that MM's tartan is less expensive than others, and an extra half yard double width isn't that much more money.