FILE : How to create a cumulative percentage by month ?

    The file for this post : How to create a cumulative percentage by month ?

     

    in this example i have

    January 60% (3/5)

    February 46.7% ((7)/(15))

     

    but i would like to have :

     

    January 60% (3/5)

    February 50% ((3+7)/(5+15))