To be honest, it is not the waist size that is the critical measure in kiltmaking, but the hips. I presume the person who posted the original question is male, but we do have women on the forum. Someone with a 28" waist and 39" hips would need a different amount of tartan for a trad kilt than would someone with a 28" waist and 33" hips. So, I always ask for the _hip_ measurement when estimating tartan.

I would also want to know the size of the sett. If the sett is small, then it might actually be impossible to use even 7 yards in a trad kilt with normal size pleats if the person is small. So, you'd get the full effect of a trad kilt with less than 8 yards (i.e., it wouldn't look like a short yardage "casual kilt"). On the other hand, if the sett were big, even a small person's kilt could easily use up 8 yards.

Hope that's helpful! If it's confusing, just holler.