To find any measurement in any pattern, divide the sts by the pattern's gauge. Take the cast on number given for the size 39 and divide it by the 6½ sts per inch. That gives you a measurement. Then take that measurement times your gauge of 5½ sts per inch and that's how many sts will give you the same measurement. Is it closer to one of the smaller sizes, or smack in between?
sue- knitting heretic