It is a good question but probably even the manufacturer would consider the answer to be based in common sense and experience. I guess the "correct" pressure depends on a number of variables. It would depend on the weight of the boat, the type and shape of the solid surface being protected against, the prevalent conditions at the dock or raft-up, and even whether they are being used vertically or horizontally. You want the fender to be soft enough to cushion a blow between two solid objects but hard enough so that it does not collapse. A fender will easily give way at the beginning and as it compresses it will continue to stiffen until it stops the force against it. Since no two blows are alike and we are not going to be there constantly to adjust pressure we usually fill them up to where we estimate that it will hold the strongest blow that we reasonably anticipate. My rule of thumb is that on a brisk wind or swells the fender should not collapse more than half its diameter. After a while you develop a feel for what is right for you and can tell by squeezing the fender whether the pressure is adequate or not. My big issue with fenders is trying to keep them clean.