The hat has an interesting construction, with a garter stitch möbius bounded by built-in I-cords, from which you pick up stitches and knit the crown. Too cool. Anyway, I hadn't done anything quite like this before, so I thought it made sense to take a test trial. I grabbed from the deepest darkest recesses of the stash some ugly blue yarn of questionable parentage but roughly the correct weight and set to work.
Like the good student I am, I first knit a gauge swatch. I counted my stitches and rows over four inches, and decided to move down one needle size.
I'll be ripping out the offending yarn tonight. I may have some trial and error in my future for construction of the hat, but I'd rather do it with a yarn I enjoy and that I can depend on. Now if only I could get some Cascade 220 in a heavier weight. . .