Customize the staffing plan details in HR module


I want to customize the staffing plan details form…like I want to change the calculations with different fields…But I am unable to do that

For example: there are 3 fields,

  1. Number of position = (This is the total number of position which can be in a company…I will enter the number here)

  2. Closed position = (this is number representing the employees present in company with same designation)

  3. Available position = (number of position - Closed position) This field should should come automatically…but i am unable to do that…

please help