Good to have a high- and low- warning that doesn't depend on batteries.
The oil lamp extinguishing at 15% is about what you'd be breathing on your average commercial flight, but then you're doing nothing more energetic than sipping coffee and admiring the flight attendant's embonpoint.
The butane-lighter test at 10%ish is equivalent to an altitude of about 16000ft, Time of Useful Consciousness given as about 30 minutes, but that's quoted for military personnel engaged in flying activities, not clambering over collapsed stope debris. Survivable but not thrivable!
Physiology plays a big part in O2 susceptibility, big No-No's are smoking, obesity and alcohol; fit and young people generally fare much better than old couch potatoes! And a hangover will do you no favours at all, whatever your body type!
The body can acclimatise over time (ie weeks) to reduced O2 levels by increasing production of red blood cells, so a really good time to go underground is right after coming back from a month in Kathmandu.
Observational comments only, no endorsement of respiratory edgeplay!!!
MARK