Crochet hat size

[MummyDuckAndDuckling]

When crocheting a hat that you know the size of head it's for, what method do you use to determine how many stitches? I was taking off 1 inch but hats are still coming up big! 2 inches seems to be a lot though..

For eg, 20 inch head, I was say 19 inches x 3 stitch (3 stitches in an inch using a 5mm hook) and using that as my total stitches.

you could try crocheting from the bottom up so you decrease towards the crown? I've got a free pattern for a simple beanie that I use all the time, it works up to 19" but has some stretch, if you wanted it bigger you could just add an extra increase round :)
www.ravelry.com/patterns/library/april-showers-hat