Formulas tend to be very exact and don't take into account every possible component group, cassette range or chain ring difference.

In reality there isn't an exact number of links your chain must be and one more or less might do better or might perform the same for a particular bike.

The big thing in sizing a chain is just knowing what to look for. If your chain sags when in the small/small then it might be too long. If the rear DR cage is stretched to it's forward limit when in the big/big then your chain might be too short.