how to get value in table column and use it in set analysis?

[achates2008]

I have a report that will calculate by context as below.

The first Open Backlog is saved in database, then latter will be calculated as: Open Backlog + Order - Shipment = Backlog = Next Open Backlog.

Currently I am using expression as : If(RowNo(total)=1, Sum(Total Netbl),Above(TOTAL [Backlog],1))

The problem is, if I select year 2018, then I will lose the beginning backlog so the result will be wrong.

To solve this, I am thinking to calculate with a way like:

Backlog = Sum(Total Netbl) + Sum({<Week={'<current week'}>} Order) - Sum({<Week={'<current week'}>} Shipment)

So is there a way to get the value of "current week" which is presented in the column "week"?

Or is there other solution to avoid the issue?