Explain to me why the solenoid would be adding more O2 than I would using the MAV to bring the po2 up to the high setpoint.
That the solenoid would be worse, assuming no overshoot which I thought I mentioned with the minimal hysteresis , makes no sense.
It is not the volume but rapid solenoid firing (i.e., volume increase) that may screw you up.
Suppose your bottom dil PPO2 is 1.0 but your high set point is 1.2. You arrive to your target depth, PPO close to 1.0. If the solenoid is set to fire automatically, the solenoid will try to rapidly make up the difference between 1.0 and 1.2. That's a lot of gas added in a short time, depending on your context, it may screw you up.
Here is an analogy. You're driving at 80mph on cruise control. You have to slow down to 60mph. Then, instead of accelerating slowly and at your pack to the previous speed, you resume cruise control and is rapidly trying to take you to 80mph regardless of what is on the road, your lap, etc. It is not the acceleration that will screw you up, it is the context in which acceleration happens.
While it is setup dependent, O2 sensors may not react quickly to O2 content changes in the loop. Therefore, the solenoid will fire many times in sequence w/o necessarily waiting for the gas to blend and propagate. That's another reason why manual addition on the bottom may help. Add O2, pull it through the loop by inhaling, adjust. You can experiment with this by having a lower PPO2 than your high set point, then let solenoid dump O2 - notice how quickly it happens and how much it affects your unit.
You can argue that you bottom DIL PPO2 is very close to your high set point and that during the descent you'll manage to get PPO2 close to the high set point anyway... That's ok, no one is arguing against that possibility. This thread, however, outlines a generic solution that will likely simplify things in most cases, including diving with hypoxic mixes which may yield 1.0 PPO2 at your target depth.