Get first previous non null column value of pivot table

[#1]

I have a pivot table as shown. User can select a month in case Dec. I have another column in this case "Previous Non Null Qty Before Dec" which I need to display the first non-null value previous to Dec for each row as shown in yellow.

The before() function seems to work, but I don't know how to determine the first non null offset. Is there a function/way to determine the offset or another way to achieve the result?

Thanks

\\

Tai