Hi Podgida, and welcome to the forum!
The psi issue is known problem in the 200 Easy Cheese book. The key is to look carefully at the context on p. 41 - just before she says, "light pressure is 2 to 10 psi; medium pressure, 10 to 20 psi; and firm pressure, 25 to 45 psi." If she actually meant psi on the cheese, that would be insanely high - 10 psi is more than most people would use on any cheese, and even for a Cantal - which infamously needs a LOT of pressure to knit - 20 psi would be very high.
But look at the paragraph just above - "In a pneumatic press, a piston is driven by force supplied by compressed air ...." Aha! In the paragraph I just mentioned above, I think she is talking about the psi of the compressed air, NOT the psi at the cheese. Unfortunately, just a bit further down the page, she discusses the calculations to determine psi on the cheese, based on weight applied to a given diameter cheese. I can't decide if she has confused the two, but it is awfully hard for the reader not to make the natural assumption that psi in the pneumatic cylinder equates to psi on the cheese - but they are most definitely NOT the same thing, UNLESS the diameter of the pneumatic cylinder happens to be identical to the diameter of the cheese.
Unfortunately, to further complicate things, she neglects to mention what size of pneumatic cylinder she has in mind. A pneumatic cylinder of 2" diameter would give a surface area of π · 1² = 3.14 square inches. So 10 psi applied to this cylinder produces the equivalent of 31 lbs of weight applied to the cheese. (How much psi that would produce on the cheese depends, of course, on the diameter of the cheese.) However, a pneumatic cylinder of 4" diameter would give a surface area of
π · 2² = 12.5 square inches (approximately), so 10 psi of air pressure would apply the equivalent of 125 lbs of weight - quite a considerable difference!
So ... the formulas get very confusing, and the books often leave out crucial information. Andrew's (Raw Prawn's) and John's advice is a bit hard for the beginner, because you don't know whether you are seeing what you should be seeing - but really, it is the best way. With a little time and experience, you will quickly find what works best for the types and sizes of cheeses you are making. One quick hint: for hard cheeses, those that go into the press unsalted, with a relatively high pH, generally need light pressure to achieve a good knit, so you need to start especially light at first (e.g., Asiago, parmesan, etc.). Cheeses that go into the press after cheddaring - lower pH, salt added - need a LOT more pressure to achieve final knit - but at the same time, you don't have to rush, because the salt has already slowed down the bacteria. So even with these it is best to start slow and work your way up until you get a good knit.