50 ml of the concentrate in a liter of 5% sulfite solution should come fairly close to D-23 1:1. I haven't tried it yet. You might need a little bisulfite to slow it down.
I don't have a proper pH meter. I haven't even tried my swimming pool pH kit. I will if I can remember where I put it.
I think it would be better to add the sulfite to the working solution than to try to incorporate it in the concentrate. I doubt it would have to be accurately weighed. 2 tablespoons of sulfite per liter would probably be close enough. I'm going to try it and see what happens. See you later.