I am planning on knitting a hat for a toddler using size US4 needles and 2 weight yarn. I am going to knit a swatch using the needles and yarn to determine how many stitches I need to cast on. I am planning on completing about an inch of 2x2 rib to start the hat and then knit in the round (making st st). Do I knit my swatch in the 2x2 rib in order to determine how many stitches I need to cast on? Or do I simply knit back and forth? Thank you for your help!