Gradebook apparently converts a letter grade to a percentage based on the number of letters in the scale when it aggregates those letter grades with numeric grades. This is not what our teachers are doing when thy convert a letter grade to a numeric for the hand calculations.
Our school uses for example, an 11 letter, letter grade scale, A+ ,A, A-, B+, B, B-, C+, C, C-, D, F
A+ 100 (100%)
A 96 (91%)
A- 93 (82%)
B+ 89 (73%)
...
F 64 (9%)
The number in parenthesis is the percentage Gradebook is using when it aggegates the letter grade with the numeric grades. A B+ is the 8th letter in the scale and is converted to 8/11 or 73%. This is not representative or consistent with the scale MOODLE allows users to define for displaying numeric grades as letters. We defined our scale as follows: A+ (97-100), A (94-96), A- (90-93), B+ (87-89), B (84-86), B- (80-83) .... D (65-69), F (0-64).

If either of the abilities below were to be available, Gradebook would match the aggregation method used by many of our teachers, namely, divide the sum of the earned points by the total possible points, a simple weighted mean approach. Either abililty would allow them to grade using a letter grade scale but have Gradebook calculate with a corresponding number of their choosing.

1. Allow the teacher to enter a numeric grade when grading the assignment but have the option of having it display as a letter grade to the student (i.e., in the email that is sent, in their user report and in the grader report. The grader report allows category totals to be displayed as either letters, value, percentages.
OR
2. Allow the teacher to enter a letter grade when grading the assignment (use the letter grade scale) but somehow tell Gradebook to use a specific number when aggregating that letter, maybe define another aggregation method, basically a simple weighted mean method where you define the number associated with the letter rather than Gradebook calculating it based on the scale. Teachers have told me that GradeQuick does just that.