I am knitting a rib-knit dress for my daughter without a pattern. I have knitted the front and the back separately and I would like some help with the raglan sleeves. I have read in Knitty .com that the number of sts to cast on for the sleeve is 20% of the stitches for the chest measurement and working through the sleeve you reach 40% of the stitches for the chest measurement. Is that correct? I have 280 sts total for the chest. Does that mean that I need to cast 56 sts on for the sleeve and reach 112 sts?