Some things to remember:
1. If the patient has a low H&H, SPO2 at best only tells you the saturation of available hemoglobin saturated with oxygen. A SPO2 of 99% won't do a patient much good if they don't have the vehicle necessary to facilitate transfer and perfusion.
2. SaO2 ≠ SPO2. SPO2 is taken via a pulse oximeter. (The formulas mentioned above need lab values, not estimates, because ultimately, the pulse oximeter is an estimate with a known deviation of error.
3. PaO2 is the heavy weight when talking about arterial content of oxygen. Period. It literally stands for partial pressure of oxygen in the artery.
4. While your patient could have done well with titration of FiO2, your rationale was flawed. It's always good to know why. RT school through the Army was much harder than Nursing school
was (personally). We spend the equivalent of a semester on hemodynamics. Some RTs don't work in a function where they use this information daily, but some of us do. Plus, I cannot speak for RTs trained outside of the military.
5. Using the oxygen dissociation curve shows us a major flaw if the physiology isn't fully understood. Notice how there is little change when the SPO2 AND PaO2 are above 95%.