Here is a site about making a custom made hat
. The gal gives some advice about decreasing, but I might add.... You can do all kinds of things with the top of a hat, but it sounds like you just want a basic well fitting rounded top. Depending on the size of your yarn you may have a lot of stitches to get rid of or not so many. You can follow the formula given, but also you can try it on (run a waste yarn through the stitches and take them off the needles if you have to) and let that help you decide. If you try it on and it is seeming about long enough and you still have a lot of stitches to get rid of, start doing it every round, or whatever to get rid of them quickly if you think you need to. Let your intuition guide you.