Multi-worksheet grade report

I want to create a view or report that stores final grades for a course section. It should compute a grade based on the following formula:

1. Take the final grades from 2 or more (3, 4, and 5 are all possibilities) sheets as input. The final grades are currently percentages, so apply the mapping:
        >= 90 --> 4
       89 - 80 --> 3
       79 - 70 --> 2
       69 - 60 --> 1
       <= 59 --> 0
2. Average these values, divide by the number of sheets and round up to the nearest integer (example: 5 sheets with 3, 4, 4, 4, and 2 would
     give (3 + 4 + 4 + 4 + 2)/5.0 which is 3.3999.. and would round to 3.
3. Display 'A' for 4, 'B' for 3, 'C' for 2, 'D' for 1 and 'E' for 0.
4. Allow user (teacher to override a computed grade on the report (example, change a 'B' to an 'A').