Pi doesn't really have anything to do with the head measurement for knitting because you measure the hat flat, not as a tube. The crown is the rounded part of your skull toward the back of the ears, which makes sense as you don't wear the hat straight down but angled. They say to knit the length from your wrist crease to the tip of your middle finger, then start the decreases, but that's about an inch too long. I usually just try it on and see where it hits me, beginning the decs when it's about 2" from the 'crown'.
sue- knitting heretic