In an unbalanced first stage the tank pressure pushes on the valve, so the intermediate pressure depends on how full the tank is. In an unbalanced piston it drops as the tank empties; in an unbalanced diaphragm it rises. The calculator works out by how much from a few measured diameters and the two tank pressures you want to compare. No IP reading needed.
If you don’t have the effective diameter of the diaphragm, a small helper works it out from its parts or from two IP readings..