I have an infant hat pattern which calls for size 15 needles and a cast on of 36 stitches. I want to use size 13 needles but do not know how many stitches to cast on. I am using chunky yarn and want to make sure the diameter of the hat is approx. 13-14 inches. I know I have to cast on more than 36 stitches but I am very confused on how to calculate this??