Need help on the calculation

hi, Anonymous 

First, you must know that calculated column and calculate table can't be affected by any slicer. 
Notice:
1. Calculation column/table not support dynamic changed based on filter or slicer, It only be affected when refresh dataset.
2. measure can be affected by filter/slicer, so you can use it to get dynamic summary result.
here is reference:
https://www.sqlbi.com/articles/calculated-columns-and-measures-in-dax/

Second, What is your data structure in basic table, If Month columns are in one field and your screenshot is based on a matrix visual, Then it could be achieved like below:

https://community.powerbi.com/t5/Desktop/Add-column-to-matrix-visual/td-p/600063

If Month columns are different fields, your screenshot is just the data structure, your requirement couldn't be achieved directly.

Since the "Month" column are just column name, It can't tell which column is the last month.

So you may try to unpivot the Month columns data, then get it in matrix visual by the above solution.

Regards,

Lin