what you're talking about is called effective cylinder pressure, cold cranking pressure, or dynamic compression. To calculate that proper number, you have to take into account things like intake valve closing point, static compression, altitude, rod length, etc. Lot's of differant facters.
Here's an excellent calculater for all that. It's for a Harley engine, but it works with any 4 stroke engine.
http://www.rbracing-rsr.com/comprAdvHD.htm