hat knit in the round -- how many decreases?
I'm trying to figure out how to adjust a pattern for a hat knit in the round, specifically, how do you figure out how many decreases are needed to close up the cap.
In most patterns, I read something like:
row 1: k5 k2tog across row
row 2 and all even rows: knit across row
row 3: k4 k2tog across row
row 5: k3 k2tog across row
row 7: k2 k2tog across row
row 9: k1 k2tog across row
row 11: k2tog across row
Cut yarn, and pass yarn through stitches, pull tight to close cap.
This is basically the pattern I want to follow, except this pattern is for a hat knit over 80 stitches. How would I adjust it for a hat knit on 40 stitches? Would I just start with k3 k2tog and hope it works? Is there some magic formula? I've consulted all my books, but have not found the answer.
Oh Knitting Sages, please help!