What ever your excel file calculates would be an unofficial rating. The rating that is published in the Tables of B16.5 are the official values...period. There are equations in the Appendices of B16.5 that explain how the values in the Table were derived, but it is the Table values that are to be used.
I have a free excel sheet I send to clients if they want it. It interpolates the ratings based on temperature, it saves me a lot time answering questions about what pressure/temp flanges are good for.