DP Macro with User Defined Horizon

Hi All,

I'm hoping this is a very simple request..

I have a requirement to sum 'x' months future demand into a separate key figure.. All very simple except 'X' is a variable value..

So for some customers they want X to be 5 months, others x is 3 months and so on..

I was thinking along the lines of a separate key figure to hold the value 'x' and is populated by the user.

similar to an excel mock up below:

(Requirement is the value 'x')

This mock up only shows one value of X per combination, but in reality X could change, eg example 1 could have an x of 6 in month 4. so should sum the next 6 months sales.

As you can see, its not too complex and I hope someone can help.