How to create a salary basis determined by simple components based on the multiple criteria?

Summary:
I need to automate the calculation of different salary components based on defined criteria. Specifically, how can I use the total salary entered by HR/Manager to derive the salary breakup (starting with Basic) using salary basis determined by simple components?

Content:
HR/Manager will enter an employee’s Total Fixed Pay. Based on the employee’s grade, the Basic Salary component should be calculated automatically as a percentage of the total pay:

• If the employee’s grade is A, Basic Salary = 30% of Total Fixed Pay.
• If the employee’s grade is B, Basic Salary = 40% of Total Fixed Pay.