Summing a variable number of cells and parts of cells

Hi, in my financial modelling I often have a calculated number of months of inventory. This number varies. I need to use this number to calculate the actual inventory value. For eg. it may be 3.2 months of inventory; in this case I need to sum 3 cells (current month, current month -1, current month -2) plus 0.2 of the fourth cell (current month -3)

But I may need to change the number of months of inventory to, for example, 4.2. In this case I would need the sum of four cells plus 0.2 of the fifth cell etc etc

Hoping I can get some advice for suitable formula that will handle the above. Thanks in advance

@Kaper thanks so much. It works perfectly