For the first exam, the mean raw score was 145 with a dispersion of 21 points.
The grading scale in dispersion units is the folllowing:
Your exams on the grading form are reported in raw units. To convert to the equivalent letter score you will have to convert your raw exam score into a dispersion score and use the above guide. Consider this a learning exercise.
Note that the average (0.00) maps into a B-