Monthly Average Usage - Excel formula

Dear all,

I'm the new comer for this group. I need to submit the report by tomorrow morning. Nee help, Just encoutner the problem for calculate the montly usage packing report. When the packing item start used at different times and sometimes they don't produce in a given month after they have started. Basically I'm trying to get a monthly average from starting month to the last full month.

Attached the excel file for my problem. Anyone to help me to set the formular, that can be automatically calculater the montly average. Thanks a lot!