When do you start to use oxygen ?
FL100-110 depending on what my sats probe says and how I'm feeling. Sometimes I start it lower if I am feeling the effects. I do not smoke and am pretty fit but even so by FL120 I am well aware of hypoxic symptoms.
I once had a couple of consultant anaesthetist friends in the back of my plane on an airways trip abroad - when one of them could no longer calculate the alveolar gas equation by mental arithmetic we figured it would be a good time to don the masks.
My ovation 2 has a very capacious O2 bottle and O2 is relatively cheap but filling a "fitted" bottle within the UK is not easy as there are not too many places with the capability to fill you.
SB