John in CR wrote:.. It was my understanding that pressure was really to adjust the ride height when loaded, with the goal to put the shock at about half travel when you're on the bike, so the wheel can move up or down for holes or bumps...
Air pressure affects the overall performance of the shock, and the sag is only a starting point for tuning it. Put a zip tie on it to see how low it will plunge during your ride. The travel should be adjusted to 2/3 of the course in average usage, so it has the last third of the travel left for unusual hard bumps and drops. You leave this zip tie on it, so you always have a reference of the shock travel after your rides. If you notice that sometimes the shock compress a tad more than 3/4 on hard bumps, your setting is about right. As Thud said, you want to use the travel of the shock as much as possible, without being at risk of bottoming, according to the terrain that you ride on.
If you want a shock to operate with lower air pressure, you can install a volume limiter inside the can, they are available in various sizes. Usually, with a hub motor in the wheel the perfect setting would be close to max recommended air pressure of the shock and damping adjuster almost completely unscrewed (set to max), unless you ride on very smooth surface. As I said previously, a shock operating with very low air pressure is more likely to fail due to bottom risk. When you store the bike for any extended period, you can lower the pressure in your suspension, although I never did and this caused no problem.
Fox publish online manuals for all of their shocks, with adequate tuning procedure explained. This is important if your shock is of a model that has many adjustment options, for tuning those may be tricky. When a shock is very tuneable, with 2 air cans an 3 knobs to play with, one has to understand its functions and get familiar with the proper tuning procedure.