john_s, I'm no chemist, but I don't think you're quite right. I'm saying add the carbonate to 200ml of water, not add 100ml of water to the 130ml containing the carbonate. By my way of thinking, that means double the solution contains the same amount of carbonate.
Perhaps changing the water to 99 parts would be more useful, but as a practical matter, I haven't seen the need to do this. But then I develop by inspection, so any slight change is compensated for in my procedure.
Over at michaelandpaula.com, there are a couple of threads on using Pyrocat HD with various minimum agitation plans. There, the use 1:1:150 to 1:1:175, so I dont' think the volume of water is critical - just something that needs to be accounted for in one's regular procedure.