The need for protein is really the need for individual amino acids, and the nutritonal value of any given protein depends on its content of the various essential amino acids, relative to the requirement for those amino acids.
This program permits you to investigate amino acid requirements by simulating experiments in which your subject is fed a mixture of amino acids containing plentiful amounts of all but the one under investigation. By varying the amount of this amino acid for a week at a time you can determine the minimum amount that is required to maintain N equilibrium.