Super cap discharge voltage level
To maximise capacitance over the lifetime of the device, you are correct to limit the voltage. Also critical to lifetime is limiting the operating temperature.

From the experimental data below, you can see that just a small change in voltage resulted in a huge loss of lifetime (in this case, 20% capacitance loss).

source: https://www.garmanage.com/atelier/index.cgi?path=public&amp;B&amp;Energy_storage/Aging/Derating


This "lifetime" of course is less relevant if a 20% loss in capacitance is not a problem for your design. Still, ESR will increase. It is application dependent.

Find how small you can make the supercapcitor before you start having problems and derate with that and your expected lifetime in mind.