Brian,
The duration figures you posted seem to make sense in relative sense. When in the normal position, above approximately 27,500 feet, the mask regulator is designed to supply 100 percent oxygen. Above 30,000 feet, you will get 100 percent with positive pressure (emergency mode), regardless of what mode you have selected. This is why the durations you listed are identical for normal and 100 percent when at 30,000 and 35,000.
You can see at the lower altitudes, your duration is significantly reduced when comparing 100 percent to the normal position.
However the magnitudes do not seem to be correct. If you have two bottles with 514 cubic inch capacity, that is only 0.6 cubic feet. Unless I am making an error, this would only supply one pilot for 48 seconds.
Best regards,
The Kid