Well you have PSI and PSIG. In the absolute the PSI is the pressure above no pressure, or perfect vacuum as we call it, the PSIG is what a gauge reads above the constant pressure of the atmosphere. So a perfect vacuum is only about 14.3 lbs negative pressure to put it in shadetree terms. PSIG is what we usually refer to. And the regulator controls the pressure from a spring inside the regulator and is also further adjusted by the engine vacuum once started. Temperature is not a factor in this deal, but it does control fuel mixture in other ways.