I'm a bit surprised that sport weight lists that as the gauge for your shawl. It's thinner than worsted and usually knits up with more stitches per inch.
When you knit a gauge swatch, make it larger than 4 inches, so you can measure the stitches between the edges without including the edges in your count. You're right, though, that gauge isn't critical on this type of knitting.
To be honest, I'd probably start knitting and see how wide it's going to be in the yarn I want after a few inches.