RD: You’re right that it doesn’t say exactly what makes for fair laws—indeed, that was the whole point of explicitly making it ‘with respect to’ something. I don’t think that makes it—in a general sense—invalid. In the same way that it’s valid that what is ‘ethical’ can be dependent on personal values, so too can justice be dependent on judicial values.
However, with both ‘ethical’ and ‘just’ (and indeed any continuous descriptor), both can be used without specifying a frame of reference. In such cases, it would be presumed that the person using the term would be referring to an arbitrary level of correspondence with their own criteria or success in the results of the proceedings in question. Strictly, the justice of a whole system is indeterminable from the system’s results regarding anything less than everything the system concerns; what generates those results must be evaluated to determine justice. (In practice, one could be reasonably confident in the justice of a system if all of its results and many of them married up with what one’s own system would dictate.)
For example, someone might say that a legal system is just even if one for every hundred people it sentences to death were sentenced incorrectly and even if it’s sentencing process differed from their own. One sees this when people say that justice has been done, even if, by their own version of justice, things would have been done differently and the result would have been different. In such cases, the judicial process was sufficiently suitable to their justice criteria that it was justice.
Your point is taken, though, that this all tells nothing of what, in practice, these ‘justice criteria’ should be. To answer also this broadly, the criteria should, to the best practical degree of success, generate effective results. In practice (and to briefly address the question as you see it), this necessitates criteria such as impartiality and pragmatism. However, I’ll avoid going into any more depth firstly because I think that it’s a distraction from my more general answer and secondly simply because I couldn’t go into satisfactory depth on such specifics. Specific and practical implementations can never satisfactorily define justice simply because they aren't definitions but examples.
