Sensitivity analysis

Start from the base case. Move one input to a low and a high value, write down the answer each time, then rank the inputs by how far they swing it. Sorted widest first it is called a tornado chart. If you can test one thing first, test the widest swing.

The trap: the swing depends on the range you choose. A range must come from evidence, such as your last four quarters or a pilot, not from "plus or minus ten percent" because that is easy.