The Importance of Knowing the PF Ratio

The PAO2 to FiO2 ratio has been used as an index of how well the lungs are working and it's been used in studies to determine when we should think about proning patients. So why would it be important for us to actually know what the PF ratio is here? I hate the P to F ratio because it's really not tremendously great. It doesn't take into account peep and mean airway pressure and it's very fidgety, but it's kind of what we're stuck with based on traditional evidence. If all things equal, no right to left shunt, normal lung per increment, if I put you at sea level on an FiO2 of