Troy,
The 40 psi is working on the area of the piston in your air cylinder, assuming your gauge is showing the air pressure you are applying to the cylinder. That force would be 40 psi x area of the piston (calculation 1, answer is pounds force). That force is then transferred to the follower in the mold. The force in psi on the follower would be (the answer of calculation 1)/78.539 sq. in. I think that's right, my math skill are a little hazy.
M