I think it isn't about specific measurements, but proportions. Measure the distance from your armpit to your waist, then measure the distance from your waist to the bottom of your butt. If the armpit-waist number is smaller than the waist-butt number, you are short-waisted. Mine is about 3" smaller (10" vs. 13" on a 5'6" woman - I don't know what is average though). And my jeans definitely look better when they sit a few inches below my actual waist - gives the illusion of a longer waist.