I was doing something in Finance and wanted to calculate the average rounded credit rating. Basically, I need to translate textual grades (e.g., AAA, Baa) to a numerical value. I found a clue in the following paper:
Becker, B., and T. Milbourn. 2011. How did increased competition affect credit ratings? Journal of Financial Economics 101 (3):493-514.
See their Table 2 for an overview of the ratings levels for the three main rating agencies and the numerical value assignments used in their empirical work.