I'd make a swatch quite a bit bigger than the stitches I'm looing for (say, cast on 50 and make about 20 rows) just to see not only the gauge I'm making, but the consistency. Also, if there is a pattern, make sure to include the pattern in there b/c that's what your garment will ultimately be.
Here's something annoying that I read
but it makes sense: wash and block your swatch before measure gauge.... :idea:
HTH,
Carie