I am a DIYer, not an expert but from my experience maintaining my hydropnuematic tank, will add my two cents.
Tank manufacturers have their own idea as to where the air level sits by how and where they locate the air volume control. Since it varies by manufacturer, I would have to say there is not one right answer for all scenarios. The theoretical ideal for max drawdown would have the tank nearly empty at kick-in. That however could become an issue if you have a softener or filter backwashing and the pump cannot keep up with the draw. Air may get interspersed in the stream and could blow resin or media into the head and into the drain.