Bungee jumping may seem like a reckless activity to some, but in truth, many commercial bungee operators rigorously ensure their equipment conforms to industry standards and guidelines. Operators commonly double and triple check the calculations and fittings before every jump.
As an operator, you need to determine the appropriate rope length, which may differ from person to person, day to day (weather conditions), and rope to rope ("stretchiness").
To prevent whiplash, injuries (and a subsequent lawsuit), bungee operators typically aim for the rope to elongate to an additional 300% (i.e. x4 normal length) for a softer fall.
If your 5'5" 145 lb girlfriend/boyfriend/malamut wants to jump and just barely glance a 47m lake below, what length should you spec the rope to be?