The Noble Lie: is it just?

Nearing the end of Book III, Socrates asks Glaucon, "Could we...somehow contrive one of those lies that come into being in case of need...some one noble lie to persuade... the rulers, but if not them, the rest of the city?"

What he is referring to as "one of those lies that come into being in case of need" is the kind of lie that he is talking about with Adeimantus in Book II. They agree that lies are useful "as a preventive, like a drug, for so-called friends when from madness or some folly they attempt to do something bad" when they are likened to the truth as best as possible. It appears that Socrates views the noble lie as a way to protect the people from the truth so as to prevent them from making mistakes or going crazy. Some advantages of persuading the people of this lie are plain: it will create unity among the people and they will love and honor their homeland. But it also creates a mysterious authority that cannot be reckoned with in the Earth as it determines (according to the noble lie) the mixture of metal in each person's soul (or the aptitude of each person). But how can Socrates hold that this noble lie would be just in an actual situation, or would he at all? This is a point of great conflation for me, although I am inclined to say that Socrates is doing all he can to make this city of necessity seem successful in being likened to the human soul. But maybe not.