My formula for nice fitting hats - gauge (stitches per inch x circumference of head) -20% = stitches to cast on. My head is 22 around, and I knit on a #7 needle 5 stitches per inch so, (22x5) - .20 = 88 stitches. Works every time. Of course in knitting and crochet gauge is everything.
The mittens I made from 100% wool.