Its called the biological value.
There is some criticism about this subject. The biological value of protein is related to the amount of protein given. When you raise the amount of protein given the biological value goes down. This is because the biological value is messured below maintainance levels. So for peopel like us it isnt the best parameter to follow.
For more info check out: THE USE OF BIOLOGICAL VALUE OF A PROTEIN IN EVALUATING ITS QUALITY FOR HUMAN REQUIREMENTS