1. Always follow the kiltmaker's instructions for measuring.
2. Don't use the measurements from one kiltmaker for a different one.
3. You're spending some cash for a kilt, make sure you measure it right.
4. Use a variation on the old carpenter's rule - measure twice. Heck measure several times to make sure you've got it right.

Another point: wear what you would normally be wearing when you do the measurments. If you would normally wear underwear and tuck in a shirt when wearing your kilt, don't strip down to bare skin to take your measurements. Some adjustment can be made by adjusting the straps, but why not make it fit how you normally wear it.