EDIT (ninja'd by JSR): Firstly, JSR is using my X system, not the old one, when he talks about an "atmospheric pressure hybrid". Second, you gave no chamber dimensions apart from wall thickness. As such, I can definitively tell you that its working pressure will be between 0psig and 500 000psig
Third: as regards the oxygen/propane system, just use GasEq to get an idea of what you're working with. Remember that lean mixes are prone to detonation and typically generate lower pressures. If you don't have a good buffer gas like helium handy, use more fuel to compensate.
Don't worry too much about the X system - with the increasing trend toward using more suitable mixes than air/propane, it's largely obsolete. In the case of air/fuel mixes, X simply specifies the number of times atmospheric pressure which the chamber is at before ignition. Since only a narrow range or fuel percentages will ignite in air, the system is largely valid for that case.
In the case of oxygen enriched mixes, buffer gases apart from nitrogen, or fuels with high partial pressures in the mix (hydrogen comes to mind), the X system is almost entirely useless. For example, using the above definition of the "X", a 1X 2:1 oxygen/propylene mix would produce over 300psi on ignition. To avert this, the system I took to using was a modified version of the existing system, where X=(partial pressure of oxygen in the mix)/3.087, which gave somewhat better comparison to the old air/propane mixes (it typically gives an exaggerated estimate of performance though).
What I suggest now is simply specifying the fuel mix as a ratio, and the starting pressure (e.g. 5:2:1 helium/oxygen/propylene). This way people can enter it into GasEq immediately and get a feel for its performance characteristics, rather than fiddling about converting from one of the two X scales to useful numbers.
If you don't have GasEq, get it. It takes very little time to learn, and is an indispensable tool to determine optimal hybrid fueling. Stoichiometric mix calculations in the simplified "high school chemistry" fashion provide a good starting point, but typically give mixes leaner than would be desirable, and don't come close to telling the whole story.
Spudfiles' resident expert on all things that sail through the air at improbable speeds, trailing an incandescent wake of ionized air, dissociated polymers and metal oxides.
