There is an advantage of the liquid concentrate when you know exactly how much sulfite you need. When you are experimenting, You must prepare the solution you think you want in order to test it. If you decide you need to double the sulfite, adding the liquid concentrate will change the total volume more than would adding the solid sulfite, or even the KOH + bisulfite.

In the meantime, I got some of the 45% w/w solution and mixed a batch with 222 grams of the sulfite solution, 22.7 grams KOH and 34 grams of Metol in water to make a liter. I'm too sleepy now, at 2:00 AM, to do any more.