It really depends on how quickly you want the crown to decrease. If you leave one plain round between decreases, it will, naturally, decrease faster and create a more rounded top. Decreasing more gradually will make a narrower top.
What you could do is determine how many more inches you want for the hat, then measure your row gauge to see how many rows you'll need to do to get to the top and decrease accordingly.
"Trust the pattern!"
Time flies like an arrow, but fruit flies like a banana.
Hop aboard while we follow the cloud-covered van of knitting harmony, man!! Woooo!! --Hedgehog
IngridKH on Ravelry