You might want to see if you can get at least a rough gauge count with the needles and yarn you're using. Those needles seem small for that yarn, at least to me. Maybe the yarn they're referring to is on the skinny end of the worsted range. I'm using US 6 with Lion Brand Pound of Love and getting about 5.75 sts/inch. This yarn is skinny compared to other worsteds I've used.
I should never overestimate my basic arithmetic skills.