*NOTE Just remember that Levi's generally don't have stretch since they are 100% cotton denim. They run a size and 1/2 small!!
The best way to find a perfect fit is to place two fingers just below your bellybutton at your lower waist(widest area) (see where the shorts sit in image 3) and Measure Here. For example I wear a modern size 27/28 or 6 and I fit a size 31 in vintage Levi's. You can also compare your favorite jeans to a tape measure, (to find the top measurement, make sure that you measure without stretching the jeans, these vintage Levi's don't have stretch)