I'm working on my first knitting top and I'm following my own basic pattern. The width of the top is 18". Using my 100% organic cotton yarn, I get 5 stitches per inch. To start my top, I cast on 90 stitches (18"X5=90 stitches). Since I am knitting the top on circular needles/aka all-at-once, I don't think I cast on the right number of stitches. Shouldn't I have double the 18", giving me the full circumference of the top, meaning that I should have cast on 180 stitches?