Some observations from my personal journey with measurement.

It occurs to me that the physics of hanging a kilt on a, say, typical 44" waist is quite different on a 34" waist. There are several factors.

First, the guy with a 34" waist is likely to have some hips -- that is hips wider than stomach, so there is something for the kilt to hang from. On a 44" waist the stomach is likely to be wider than the hips, and so the kilt needs to be clamped to the stomach to stay up. Clamping implies some pressure, and this will reduce the waist size, or rather a smaller waist is required to ensure sufficient clamping pressure (else the kilt must be cinched by a belt).

I used to be 44"-46" range, and found that a kilt purchased at my precise relaxed waist measurement would just fall off as soon as I moved. I needed a bit of clamping pressure to keep it in place. An issue here is that a 34" waist varies a lot less as the body moves than for a 44" wearer.

The 44" waist is associated with a fair bit of pliable fat around the stomach. When I was gaining weight, the fat was denser, and needed less clamping force. When I was losing weight, the fat was less dense, and needed a lot of clamping force. This really foxed me when measuring up.

Another issue is that stomach fat droops. Women have a more extreme version of the same problem when measuring for a bra size. The bra size is based on how far the breasts protrude from the chest, yet for many women the breasts droop. For women, bras reshape the breasts to a more appealing shape, quite different from the natural hang. In a similar, but less dramatic, way the kilt acts somewhat like a corset, reshaping the stomach fat on a large guy. So the size required is the size of the reshaped fat.

Thus I find measuring a kilt, as a larger waisted male, to be very complex. I've discovered a few tricks that work for me. First, I measure my stomach lying on my back, completely relaxed. I don't have to mentally decide how relaxed I am being in my stature, nor worry about my podge drooping uncharacteristically. Second, I look at my pants. I ignore the stated size, as it's usually wrong, but measure the actual size of the pants to get an idea of what degree of clamping I find necessary to keep my pants from falling down. If I know a 44" (actual size) pair of pants falls off me, then why shouldn't a 44" waist kilt fall off me also. (This is imprecise as the waist can be a different points, but can still provide useful information)

All this has tended to mean that I've personally bought kilts an inch or two smaller than a simple 'let it all hang out' waist measurement would imply. However, as I am now about 38", this effect seems to be declining.

Your mileage might vary.

KP