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.