If your gauge is off with the recommended needle size for your yarn, I've found as a "rule of thumb" that changing the needle size 1 size* changes my gauge 1/2 stitch per inch. That way, if I'm waaay off, I can move several needle sizes at once rather than reknitting the gauge swatch 5 times.
*that means 1 size, so 10->10.5 counts as 1 size, since those are both needle sizes ... is that too pedantic? oops...
(posted by me this day on spindlers)
