Data is only redundant if the data isn't used
Data is redundant if it exists in more than one place. Whether it gets used or not from all of these places is irrelevant.
as I said it would be possible to automate the grading system on both creation of a DR and editing of a DR.
That sort of automation is really just asking for trouble. Materialized query tables is a standard DB feature, meant exactly for this kind of situation - derived values that do not change often.
Data such as grades are nothing compared to things like images, and I doubted the server will miss then 423bytes it would take to store single character grades in a database.
It's not an issue of space. It's an issue of maintainability, complexity, and logical consistent DB structure.