When you set up the forecast plan in MP, one of the attributes is called the “reduction principle”. That attribute allows for various options to tell MP how to automatically reduce the remaining forecast.
To tell it to ignore any remaining forecasts amounts at a certain number of days, (dynamic/rolling as opposed to a specific fixed date), you’d use one of the “reduction keys” options and then set up a reduction key rule accordingly.
These have pretty straightforward help screen texts, but if you need additional details that could probably be covered within this thread.