Formula to calculate forecast

Hi everyone.

I have a formula which works with a number column (Duration) and a start date. The formula takes into account each month to see if the month is during the duration of a project. If it is, we are then calculating the monthly cost for a project. However, this monthly figure is based on 20% of the project total. The remaining 20% will be paid on the month after it ends. So if the last month is August, then the 20% will show in September. Can anyone help me with how to adjust this formula to show the 20% in the last month?

Formula for calculating if month is in duration:
IF(AND(
FORMAT_DATE(DATE(2024,1,DAY({Start Date})),"YYYY-MM-DD")>={Start Date},
FORMAT_DATE(DATE(2024,1,DAY({Start Date})),"YYYY-MM-DD")
<FORMAT_DATE(EDATE({Start Date},{Duration}),"YYYY-MM-DD")
),{Product Value}/{Duration},"0")

Below is an example of the board I’m working on.

Thanks for your help.