Sorry mr Checkboard, Oxygen pressure not volume, is checked prior to dispatch and the temp corrections for pressure are tabulated. Engineers fill to a agreed service level and I can assure you that the volume does effect the pressure, hence the two tables 76/114 cu ft. Obviously pressure will depend on when it was last filled as we do pre-flight oxy checks every duty start and it is bound to deplete, as you observed, temp also effects.
Oxygen systems and flows are approved according to the flow rates set down under ICAO, FAA proceedures based on average person, average workloads and then adjusted for demand, low, normal, high 100% etc.
Boeing have an estimated rate: Sedentary 14LPM
Normal 24LPM
Severe 40LPM
As defined in the Boeing operations manual.
Obviously at the pre dispatch stage we look at the route and invariably, 30 mins Oxygen will suffice. We base this on high demand rate, being the most restrictive. ( Boeing manual calculates 25 mins )