How is it going? I find that my gauge is different when knitting in the round, but not a lot different. I have tried the trick Orangeus mentions. It sounds like it should give an accurate gauge for knitting in the round, but I find it doesn't really work for me. I don't know why.
I do a lot of hats and size is an issue but doesn't have to be as critical as with some things that have a "fit". You could probably just use your gauge from working flat. I find with a lot of things I do that even though I do a gauge swatch, when I actually knit the piece my gauge changes for some reason.
So I do big swatches sometimes.
By that I mean, the actually piece turns out to be my gauge swatch. At least hats aren't too big when you have to do them over.
I hope you are having good luck with the hat and the gauge.