There isn't a way to use the percentage correct in OG to figure out what kind of GMAT score you would get - they're not at all the same thing.
Have you taken a practice CAT? If not, that's your next step (you say you have one of our books, so you also have access to our practice CATs). If so, and if you took one of ours, then you have the data you need to assess your performance on RC relative to the other areas.
Here's how!
First, read these two articles:
http://tinyurl.com/executivereasoninghttp://tinyurl.com/2ndlevelofgmatThink about how what you've been doing does and doesn't match up with that and how you may need to change your approach accordingly.
Then, use the below to analyze your most recent MPrep CATs (this should take you a minimum of 1 hour):
http://tinyurl.com/analyzeyourcatsBased on all of that, figure out your strengths and weaknesses as well as any ideas you have for what you think you should do. Then come back here and tell us; we'll tell you whether we agree and advise you further. (Note: do share an analysis with us, not just the raw data. Your analysis should include a discussion of your buckets - you'll understand what that means when you read the last article. Part of getting better is developing your ability to analyze your results - figure out what they mean and what you think you should do about them!)