90-94% is fine, generally.
There are various views on this, and there is a lot of variation within the medical profession also, but in general you want to keep your oxygen level no lower than 10 percentage points below your sea level figure (which for most non smokers is about 98-99%).
One can tolerate lower levels for short periods; for example 85% might be OK for half an hour but most people will arrive well knackered after such a flight lasting 5 hours. This is why I use o2 anytime above FL090, and try to keep it above 94% all the time (not easy with a cannula at FL180
). Many Eurocontrol IFR routings are poor below FL120/130 anyway.
Your brain doesn't stop working at say 85%. It is a gradual thing.
There is a large subsection of the IFR community which flies without oxygen, works hard to find routings at/below FL100, and wonders why they are so tired after every trip.
What kind of oxygen system are you using?
Some notes on oxygen systems are
here.