Circular Ref - But using prior month value

 Hi, I am trying to build a module which needs to take the cumulated value of a line item from the previous month and reuse it in the current month. This is not circular as the time offset means it is not self referencial.  However Anaplan gives me a circular reference error. I have attached a small working to show what I am tring to acheive and would appreciate any advice on how to fix and explinations on why it is happening so I can avoid this in the future. Many thanks