Ann, Cantal is a special case -- I would think it wins the award for the hardest-to-press cheese. By the time that it is pressed once, milled, salted, and then, 24 hours after being made, pressed again, it has absolutely zero inclination to stick together. Thus, when I made a 6-gallon Cantal, I went up to 900 lbs (!) for 12 hours before it really began to close up the rind completely. That was using an 8" mold, so let's see ... 4² * 3.14 ≈ 50 in², and 900 lbs. ÷ 50 in² = 18 psi.