From what I can see from the ads for the pattern, you can knit this hat in any yarn and gauge. So determine the gauge for your yarn, then select how many stitches to cast on from that measurement.
To check your gauge on a circular needle without making the whole hat, you can cast on about 5 inches and work across. Instead of turning the needles, slide the stitches to the other end of the needle, let the yarn drape loosely across the back, and work the next row. This way you're knitting the way you would in the round--only on the front.
"Trust the pattern!"
Time flies like an arrow, but fruit flies like a banana.
Hop aboard while we follow the cloud-covered van of knitting harmony, man!! Woooo!! --Hedgehog
IngridKH on Ravelry