To calculate the pressure for any non-given value, use this equation, but note that the pressure can't go above 800 PSI (the vapor pressure of CO2).
4200/Chamber Volume In ci=Chamber Pressure in psi -OR- 4200/CV
-From CO2 Cartridge in the wiki.
Where does the constant, 4200, come from? I have a class tomorrow and plan on bringing in my launcher(filled using 12g CO2), but I would really like to be able to explain as much of the physics as possible. If anyone knows, I would greatly appreciate the information!