I've worn a mask at FL250 and kept my oxygen sat level over 95%: it wasn't a demand mask. Several folks I've spoken with have gone to FL290 on a non-pressure mask and kept sat. above 90%. We're all different and there doesn't seem to be complete agreement on where reflexes and reasoning are affected. 90 percent is a little conservative for me personally, but a great minimum target to be safe.
On cannulas--oxygen conservation ones from Scott, my sat level drops below 90% above 16,000 unless I breathe deliberately from my diaphragm--deep breaths. So, I go to a mask about 16,000.
At night, I go on oxygen above 10,000 and most of the time do that during daylight if I'm going to stay that high for awhile.
I've functioned effectively below 90, 85 or 86 seems to be were my reactions slow. I've done several tests on several occasions with a co-pilot. Doesn't mean I don't get a headache later though. Therefore, 90 is a great minimum.
I believe Mountain High is now advertising a pulse cannula certified up to FL, but since I'm going to a pressurized plane, haven't checked the details.
Best,
Dave