I'm just going to add my bit to the above - which are all good opinions. However, I can see you really want a definitive answer. So:
It actually comes down to each car. Now you ask what part? Well, its the whole thing, including the driver!!!!!!! I've developed a simplified formula for you:
The second page got beer spilt on it so I decided just to skip to the last equation:
RB20DET ~ 10-15psi.
Hope it helped :laughing-smiley-014: