Try the gauge swatch with the needles you think you could use. If the swatch is too big, use smaller needles. If it is too small, use larger needles and make another gauge swatch till you find a needle size that will make a gauge swatch of the closest size to the correct gauge. You have to play around with it to get the correct match.

Hmm, the gauge specified in the pattern seems to be DK or sport weight which usually uses needles smaller than 9. Is the garment open and lacey? The yarn you plan on using sounds more like worsted or heavy worsted and if you try to knit it on a smaller needle to get the correct gauge, you may end up with a very stiff fabric. You could mathematically figure out how many stitches to cast on based on the desired width of each piece but it would be dicey with shaping, bindoff for armholes etc. Can you find another yarn of similar weight to the yarn called for that you like?

Yes threejsmom....exactly what I was thinking...the garment was made with a knotty looking yarn called bubble...I made it for the first time about 16 years ago...and really liked it. Now, I'd like to use the smooth look instead of the knotty one for the second one I'm making...however, someone game me this yarn and I love it and would like to use it....instead of large maybe I could make a small size.
How can one mathematically figure this out...math was not my forte....lol?

Hi, all. This is my first time posting on this forum, usually I'm at Kitchens or Home Decorating, but I knit all the time. Here's a link to instructions on how to change sizes or gauge on an existing pattern. It is just simple math.

Here is a link that might be useful: calculating size or gauge