The fit of a hat depends very strongly on the stretch of the fabric. When I designed the interlock hat pictured at
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folded the edge of the fabric to simulate a hem -- the hathas a very deep hem so that I can also wear it as a watchcap-- and pinned it around my head to determine the size. Then I added seam allowance and drew the pattern right onthe fabric. (Drawing on the fabric won't be an option withfleece, I imagine.) The main part of the hat is a simplerectangle, then I drew four right-angle triangles across thetop, to sew together to make a square top. I was hoping tosuggest a medieval scholar's cap, but the corners hardlyshow at all -- I should have cut four panels that spread outat the top.
If you cut six equilateral triangles, and round the corners of the "darts", it should make a nice domed cap. The number of darts should be even, so that you can sew the side darts first, then sew up the seam and two of the darts in one operation.
Try the hat on before sewing the hem, in case you want to make it wider or narrower than planned.
Joy Beeson