>>simmer+A9
They have an excel sheet next to it - they can test it against that. Plus they can ask questions if something seems off and have it explain the code.
>>martin+S9
I'm not sure being able to verify that it's vaguely correct really solves the issue. Consider how many edge cases inhabit a "30 sheet, mind-numbingly complicated" Excel document. Verifying equivalence sounds nontrivial, to put it mildly.