Math? Adjusting the width of a rectangular shawl/scarf HELP NEEDED

[boo1]

I posted this on "How To" but maybe it should have been posted here.

Here is the pattern:

CO 50 st. on size 13 needles

Knit 3 rows.
Pattern: K2, *yo, k2 tog*, k last two.
Repeat pattern stitch to desired length.
Knit 3 rows.

If I want to make it narrower, say fewer than 50 CO, what number do I use a mulitiple of?

Thanks in advance, and sorry for double posting. I'm leaving in a little while, haven't packed (except my yarn and needles!) and have to run to the doctor's office first.