The leather straps that are used on most standard kilts allow for 4" of adjustment. In my experience, most kilt makers make the kilt so that it fits correctly on the first hole in the strap. Besides the fact that more men are concerned with weight gain than with weight loss, there is another reason why they do this.

Speaking as someone who also does kilt alterations, if I have to take a kilt in by a couple of inches, this is easily accomplished by simply changing the placement of the straps and buckles. However, if I have to let a kilt out by a couple of inches, I typically have to completely rework both the inner and outer aprons. If there is not enough cloth in the hem of the aprons, I will need to go in and remove a pleat, so I can work from the other side of the apron. And letting a kilt out also usually involves replacing the lining, and replacing the waist band. In other words, it's a heck of a lot easier to take the kilt in a little bit than to let it out.

So I think that's why most kilt makers want to allow as much room for expansion as possible when they initially make the kilt.

That being said, if you seriously think you will be loosing a couple of inches from your waist in the coming months, go ahead and plan for it. Tell the kilt makers that you want your kilt sized to fit in the center hole of your straps. But give them your actual measurements. Don't try and guess yourself what measurements you'd "like" your kilt made to. Just give them your real measurements and tell them "I'd like this to fit at the middle hole of the straps, because I plan on losing some weight," and they should be able to accomodate.

Aye,
Matt