you can always just get the nylon knee highs from walmart ($.33!!!) cut off the thigh part and sew ends together for a stretchy headband no measurements needed.
They are perfect for newborns since they are soo soft and fit so well! HTH
heres a pic of the recent one I made...shes only one which justifies me saying it streches well! it will fit just at perfect on a newborn-Again think NYLON!
you can dye them any color as well to match bows!!!