I have a matrix with region related fields as the column headers.  A slicer for this matrix contains about 40 regional related fields available to populate the matrix columns.  I have instructions on the slicer telling end users to select only two regions from within the slicer.  This means any 2 of the 40 available region related fields can serve as coulmn headers.  My rows are populated with item numbers.  Currently my matrix shows the List Price for each item number in both of the chosen regions, see example.  I need a fourth column that calculates the difference between column 2 and column 3 no matter what two regions are selected from the region slicer.  I have tried a formula that relies on the Allselected function and an [Index] but the [Index] value goes unrecognized by Power BI.  

Here is the calculation that is not working, note the error bar under the [Index] instances.  Should I have a separate column for [Index] or can I define [Index] within my calculation?  Is there a way to define column 2 and column 3 and then just take the difference no matter what the header says for these columns? 

Its frustrating when I can do something in Excel but not PowerBI.

 

 

 

EDIT:  I would ike to add that I am open minded to a solution where a user picks a region from one slicer, a second region from a second slicer and the matrix shows the list prices and any difference.