Fast formula to get the prorated grade maximum from a Value Set

Summary:

Hi Experts,

We want to create a Fast Formula to prorate the 'Maximum Bonus Opportunity'. For that, we created a DFF in employment info with a Value Set that allows the local HRs to input the number.
The idea is to prorate the value in this DFF throughout the evaluation year.
Here is the case scenario: John (random name) has the Maximum Bonus Opportunity 1.0 from 1st Jan 2024 until 30th June 2024 and 0.8 from 1st July 2024 until 31st Dec 2024.

The expected result is 0.9 since John has both values (1.0 and 0.8) for 6 months. However, the value the formula is giving is 1.

What are your ideas to make this work? Really appreciate your time!