I usually reduce my ribbing needle to correspond to any change in the regular needle. That's the easy part.
If you're off by 1/2 a stitch for 2 inches, I don't think going down another size is going to really handle that. I think you'll get too tight.
Measure your swatch in 3 spots and average them. That will give you a better overall gauge. Then measure again as your knitting, since it can change a bit then, too. I find that a swatch is often tighter for me than the actual garment, since I relax more when I'm just knitting away.
Then there's blocking to consider, too. That could take care of any minor discrepencies.