Boeing quote a dispatch pressure in PSI not volume as clearly, a larger vessel will have a different pressure to a smaller one,
I don't think so! the pressure in the cylinder depends on when it was last serviced, and to what pressure it was filled.
To covert the size of the bottle to a time, multiply cu ft x 28.31 to get litres, then divide by flow rate, on 100% it should give you at least 30 mins.
Again not quite.
gas consumed = breathing rate x time x ambient pressure
The "114 cubic feet" refers to the amount of gas in the bottle(s),
at sea level standard temperature and pressure when filled to the rated pressure for the cylinder. At altitude the volume will be higher.
The breathing rate or respiratory minute volume (RMV), in litres per minute (lpm) in normal conditions will be between 10 and 25 lpm. At times of high work rate or panic, breathing rates can rise to 100 lpm.