Welcome to the boards! This is pretty fun place to "hang out" online. Lots of good natured folks on here, and typically with a great sense of humour. If you must measure, use what ever system the maker uses, simplest answer... which is usually the better one.
Of course, you could always take things back a ways, and go with belted plaid, or feilidh beag, and cut out the worry of measurements all together (more or less), and if you gain some weight, or lose some, it'll make very little difference... just change your pleating!
Lots of variety... depending on what you are interested in. Heh.. .happy hunting!