I re-read your first post. I think that you want to select the prior month from the current month of run time, then rename that column to something static for formula use, then return it to the original column name. I would need a workflow to try and tackle this as it will require a few steps and column name variations could require different method approaches. Here is something to get you started.
You can get the current date info at execution time using this approach.
Then Date & Time Shift set to Month and -1
Then Date & Time Extract to get the month and year in a separate column.
Then Column Expressions to make the month and name formats match your column names / combine them so they match your column name format.
Then convert that to a flow variable to and use it with the Column Renamer to switch to a generic name
Then use the same flow variable to return to the original name using a Column Renamer after your formulas.