The Church knows exactly what is meant by a just wage.
It knows the characteristics of a just wage, that is all (and you probably agree with that), not a number.
As Pope Leo points out, there are far too many variables. The Church promotes collective bargaining to arrive at a just wage.
That's a job for economists.
They can't arrive at an answer because there is no answer. In my book I give the example of an independent wheat farmer and the miller. The wheat farmer, William Farmer, produces 2 sacks of hand ground flour a day. The miller, Chaim Goldstein produces 150 sacks of flour a day using his automated farm and mill. Chaim realizes if he hires William he can increase production to 200 sacks a day.
I then challenge the reader: You are in a distributist economy and head of the farming soviet. Your job is to come up with the right wage, in sacks of flour, that Chaim has to pay William. I then show that the "right" answer could be 3 sacks of flour (50% raise) to 199 sacks (Chaim doesn't have to go into the fields anymore), or anything in between. Without a market you can't determine the right answer (the calculation problem). Come to think of it, William might be happy with 1 sack of flour a day in exchange for Chaim doing the paperwork and the fact that William can do the job in an air conditioned combine.
An economist also can not determine the right answer. What we have is the question on the proper way to allocate production between owners of capital and labor. The market does this, kind of like an optimization engine (it is never "right", it tends towards the optimum).
Pope Leo seems to recognize this and recommend collective bargaining, which gives negotiating power to labor. Note I know with the horrors of unions in the past many recoil from this, but economically Pope Leo is correct, and this is a good way to prevent undue influence from the State and let the market work it out.
As far as unions, we need reforms, but right now in a right-to-work state my go-to contractor is union. We freely choose to use them. So I'm speaking from first person experience, collective bargaining can work well, in fact it is great, if we learn from the past and fix the problems, e.g. work rules rackets.