How to prorate upfront absence plan

Summary

We have an annual leave absence plan, where employees are paid the entitlement upfront. The challenge we have here is we need to prorate the entitlement if there is any change in the worker assignment

Content

Hi All,

We have an annual leave absence plan, where employees are paid the entitlement upfront. The challenge we have here is we need to prorate the entitlement if there is any change in the worker length of service (LOS)

For example anybody below 5 years should receive 12 entitlement and remaining 24 

So if employee's LOS changes from 4.99 to 5 mid accrual year his entitlement should prorate.