I am of this opinion, that too many recipes are wrong, when it comes to mention about weight/press. As an example: Press with 15 kg. Then the PSI will depend of the size of the follower. The bigger the follower, the less the PSI. All recipes should mention the PSI, and not the weight.
David: Think, you mix it up a bit. You cannot say: 15 kg. PSI. PSI means: Pound per Square Inch.