Answer

1. Create a Calculated Field [LookupValue]
   
   LAST()
   
   What this does is Index the columns, so that the column on the far right is always 0

2. Create a Calculated Field [Across_Value]
   
   WINDOW_SUM(IIF([LookupValue]=0,([Value]),0))
   
   What this does is check each column for the value of LAST(). If it is 0 (The last column), then we sum the Measure amount, else, we have 0.

3. Create a Calculated Field [Rank]
   
   RANK([Across_Value])
   
   What this does is Rank the SUM of the column on the far right in Tableau.
   
   For the COMPUTE USING for RANK
   
   Rank > Table Down
   Across_Value > Table Across
   Lookup > Month Then place the [Rank] field on the worksheet on the far left as a discrete measure, and hide the header.