Macro

I am a begginer is useing Power BI and DAX. I need to substruct within one column. The column is called "DR" and has only DR08 and DR07, but this will be growing each month, so next month I will have DR09. I need to subtract every month the current one (so it will be DR09) and DR08, then in September, it will be DR10 and DR09. Is there a way to write a macro (or other solution) that will always take the current one and substruct from the previous one? Each dR comes from different file, and Forecast table has all them append.