Your math looks right. Does the pattern have any rows before and after the charted part, i.e., a border? As for your gauge, you've just made an 8" swatch, and it's shown that the gauge is off. It happens. Is your yarn a true worsted weight? What gauge does the ball band list? Yarn weights aren't exact, so you might have a worsted that's on the thin side. If you maintain the density you need, do you have enough yarn to do an extra repeat, i.e., an additional 18 rows?