HSA calculation for Employee and Employer contributions

We have a requirement where the HSA calculations should be changed:

Existing –
1.    The maximum limit of HSA Total will remain as per the IRS stated limits.
2.    Employer contribution is fixed irrespective of the employee’s date of joining.
3.    The maximum limit of Employee contribution is prorated based on the employee’s date of joining or change in the coverage limits due to a life event.
4.    Total = Employee contribution + Employer contribution

Required –
1.    Employer contribution should be prorated based on the employee’s date of joining or change in the coverage limits due to a life event.
2.    The maximum limit of HSA Total will remain as per the IRS stated limits.