You might make a template data source a user can fill out (for example a Google Sheet form, or an Excel workbook), and then connect that data source into the final workbook. You should need to specify every possible variable they could specify, and then you would have to include those variables in a calculation in the workbook.
I'd definetly be plugging the parameter - this is how you are going to manipulate your calculations.
Once you have them setup with the +/- values, simply create a calculated field;
You could also modify your logic for weighted calculations etc...
([south measure] + [South Parameter] / [original measure]) + ([north measure] + [North Parameter] / [original measure])