Student Grade Calculation Formula Development

I need a formula to calculate student grades based on a letter grading system. The grades should be calculated as follows:

- A: 80-100
- A-: 70-79
- B+: 64-69
- B: 58-63
- B-: 52-57
- C+: 46-51
- C: 40-45
- C-: 36-39
- D+: 32-35
- D: 28-31
- D-: 25-27
- F: 0-24


Ideal candidate should be proficient in mathematics and understand grading systems in education.