Whatever "gauge" needle size, I go down 2 sizes for "ribbing".
If the pattern "suggests" an 8 for the body, and a 6 for the ribbing...that is two sizes...so I follow suit. It's always worked well for me!
Hint: sometimes it is good to re-check your gauge after knitting up for about 10 inches. Some knitters start with sleeves for that very reason. To check gauge after 10". With sleeves, it's less to frog if the gauge doesn't hold.