The Rating Score is simply a sum of all Graham Ratings. For example, a score of 4.5 indicates that the stock clears four Defensive ratings (including any Utility / Financial ratings), and one Enterprising rating.
It's quite easy to search for any combination of Graham Ratings using the Advanced Graham Screener, such as all stocks with total ratings above 8. But the Rating Score was only introduced as way for users to quickly understand a stock's position, and has little relevance to Graham's actual framework.
Graham required specific combinations of these ratings, such as the rating combination for defensive stocks, which can be more simply screened as follows; or on the free Classic Graham Screener as follows.