This description doesn't quite make sense. If the threshold is regularly changing, the calculation can output the same result number for the same 21 parameters

This description doesn't quite make sense. If the threshold is regularly changing, the calculation can output the same result number for the same 21 parameters and have it be a bug or not a bug from month to month, depending on the threshold. How can you write a test for that without locking in the threshold? Indeed, without hard-coding the threshold in the calculation itself?