Cracking effort is given as a pressure, a few millimetres of water, but it comes from forces: the intermediate pressure pushing the seat open, the bias spring holding it shut, and the lever and diaphragm that let your breath overcome the difference. This calculator follows those forces through an unbalanced or a balanced second stage.
Fill in what you know and leave one field empty; that is the one it works out. Leave the cracking effort empty to get it from the parts, or enter the cracking effort you want and leave the spring, lever or diaphragm empty to see what it takes.