If we are talking about how an individual might rationally develop a moral system, then maybe reflective equilibrium can help. Perhaps we can agree that the individual who has developed such an equilibrium has good reason to accept certain moral principles and judgments.

So far so good, but there seem to be limitations to this method. 

• First it is only a consistency check. Two individuals could go through the process of developing an equilibrium and come to very different results. 
• Second, the method appears to rely heavily on our prior moral convictions. I'm tempted to say that it is strongly biased toward our prior moral views, whatever they are. I can imagine slave holders developing an equilibrium that allows for slavery and abolitionists developing an equilibrium that forbids it. Where does that leave us? 
• Third, many of the philosophical notions that are used to develop the equilibrium may be quite controversial in themselves. If that is true, they will not help to settle disputes.