Wayne...I built a similar version of your press. Works like a charm. To prevent the follower from twisting or moving I made sure it was level. I also added a couple of 3" screws to the bottom and top of the wall stud.
The arm weighs in at 25#, so I used weights in the beginning and moved to the arm with some #10 cans of tomato sauce. I will make up some water jugs in the future to I know my weights are consistent.
Now, for the conversation about psi or pounds.
They are the same thing in this regard.
If you have 4psi on a 4" mold and you convert the recipe from a 2Gal to a 20Gal, that would go into a much larger mold. When you go to press the curd, you are going to need the same "mass"/weight across the complete surface of the cheese face.
Also if you were to take an industrial recipe and make into something smaller, you would have to convert psi to pounds in reverse.
Your one common factor here is the pounds per square inch. If you have a 4" mold or a 400" mold you will always have to apply the same PSI. With some "simple" math one can figure out how much more weight is needed to apply the correct PSI.