I’ve seen quite a few head sizing charts referring to circumference. This is easy enough to measure from someone’s head but not so easy on a crochet hat. A lot of the charts also contain too much information for my liking.
What you really need is a guide to the diameter you need to keep making increases for. Once you reach the suggested diameter measurements in this chart (no real need to be exact, I round up or down depending on the frame of the subject the hat is for), continue with no further increases. This method works no matter which stitch you’re using. Enjoy!