Help adjusting pattern please

I am looking for a pattern to make a scarf with some elann pure alpaca - suggested needle is size 6. While I can knit, I am not so good at altering patterns. I would like to make this scarf, but will need a little guidance as to # of stitches as my guage is 20sts/25 rows = 4"… so if I cast on 45 stitches should I be good? That would give me 8", but I figure the ribbing will pull it in a little smaller…

Hopefully this link works :

http://www.classiceliteyarns.com/pdf/sinfulribbedscarf.pdf

From the look of the pattern, the width of the scarf can be anything you want it to be, as long as the stitches cast on are a multiple of 6 plus 3. 45 stitches, with your gauge, might be closer to 9 inches instead of 8 or 7. The next smallest option is casting on 39 which will probably be around 8 or 7 inches wide.