(physician here) Vents can be quite tricky to get right; primarily because as disease progression continues on many things change. We are always trying to balance the deleterious effects of the intervention (in this case, the vent) with the goals of care.

What most people don't know is that vents cause a lot of injuries on their own. (See ARDS). Hypoxemic respiratory failure is serious stuff, and it's almost always not the only insult we're dealing with. So, it's about balance.

In the feedback loop is a lot of things. Certainly, oxygen saturation is a core measure, but we are also thinking about things like hypercapnia (and acidosis); overall perfusion status (blood circulation); infection management; etc.

Gas exchange is important, but there are a lot of things that have to go right for the lungs to take oxygen in, get it into the right transport, and get carbon dioxide back out. You can over-optimize for any of those at the expense of the other (see: hypercapnia).

In short, it's complex and there isn't a solid formula that will work for every patient. It's actually the subject of a lot of debate on details (some of it quite passionate in our Critical care / pulmonology community). This is why we call it PRACTICING medicine ;)