I've been waiting days to use that title.
The Santa Cruz hat from the latest MagKnits is knitting up well. It's a lace design, but not a very complicated one. If anyone's looking to an introduction to lace knitting, this here's your pattern.
When I did the math, I discovered that Sarah-Hope's sizes were a bit small for my head. Her pattern results in a hat for a 19-inch head. Mine's 21.5 inches. Further investigation uncovered that the number of stitches cast on (if one were to pretend that this was a stockinette-stitch hat) would result in a hat circumference of 15 inches.
Sarah-Hope's doing this because she knows lace stretches. But, I decided to add on one more repeat to accommodate my big head. I cast on 84 stitches instead of the pattern's 72. This means that I need to fiddle a bit with the crown decreases to accomplish more decreases in the same number of rows. Instead of Sarah-Hope's interval of nine stitches between the first set of decreases, I'll begin the decreases one row later, and use an interval of seven stitches.
How did I arrive at my alternative instructions? I fudged. There was no formula and no math applied. Trust me.