Do you mean Studio Finish concealer, the one in the eyeshadow looking pot?
If so I'd suggest getting NC20 also. But it's best if you can to go to the counter or store and be matched up by an MA before buying.
I'm NC35 in Studio Fix Fluid Foundation and I have found that NC30 Studio Finish concealer is way too light. It actually starts to enhance whatever flaw I'm trying to cover up cause it's lighter than my skin tone. I've found that NC35 Studio Finish concealer works best for me.