Can you post a picture? There's a link in signature about appropriate sizes and how to.
Based on what i think you mean...There is no way to do it perfectly, but it seems to me that the best way in this case will be to decrease quickly rather than the slower rounder top. I'm thinking k2tog all the knit ribs, then on the next row p2tog all the purl stitches. Then if there aren't too many stitches just thread the tail through and cinch it up. Make sure the hat is long enough if you try this because this type of method shortens it considerably.