Hi,

I understand you about your instructor issues. I had a very experienced instructor but following his course recipe. You had to adapt to him not the other way around. So I checked with other expert people and my personal experience and I apply the following:

While in training changing to high set points too early and your limited buoyancy control at this level could provoke spikes in your PO2 due to very fast descents. I find change around 60 feet/18 mts comfortable enough. And avoids the continuous injection of O2 that adds difficulty to the already difficult buoyancy control for a newby.

As you are getting experience you have more control on descent and you can change earlier, but you have to consider that changing your set point has to do with your decompression requirements. As soon as you change to your high set point less deco requirement. But if you convert the PO2 to Open Circuit equivalent O2 percentage, you can figure out what is reasonable.

For example 1.3 at 60 feet/18 mt is equivalent to breath nitrox 46 and 0.7 at the same depth is equivalent to nitrox 25 so sound very reasonable this change at this depth. Earlier change don't have so much deco advantage (we are talking minutes here) and avoids the continuous injection of O2 and its effect in buoyancy.

For choosing the best high set point value you have to consider O2 toxicity versus deco requirements. You should have at least 1.0 (deco efficiency) and not more 1.3 (oxygen toxicity). If your dive is short with a mild deco you can go up to 1.3. If your dive is very long with a long deco you should go down to 1.1 probably. You can compare the OTU's and CNS toxicity with software like V-Planner to make your decision.

In my way up....I had more problems there. My first instructor force me to use the high set point during the ascent. The reason behind is that if you go to a low set point and you go up too fast your PO2 could drop down dangerously. The ascent in this case is probably the most difficult skill to master on a rebreather. I struggle with my ascents for many dives until I met my deep and cave rebreather instructor. He told me he changes to a low set point at the beginning of the ascent and keep the PO2 manually during the ascent. In case the PO2 drops bellow 0.7 then the injector will start working assuring a minimum life sustaining PO2.
I tried and it was magic... I add one or two pushes of O2 manually and I exhale by my nose a little bit at the same time. Buoyancy don't change and I can control much better my ascent rate so reducing the risk of a rapid ascent and subsequent drop in PO2. Once at the deco stop depth as you were keeping your PO2 manually you fine tune the PO2 manually and if the deco is long you can change to the high set point manually to relax a little bit.

I hope this help you, at least is what I do and works fine for me.