I have fought this argument for years.
WHen I use manufactured powder chemistry instead of what I make myself, I always mix it by the spoonfull and have never had a problem with inconsistency of processing, or any other related to using incomplete chemistry. I mix the powder by shaking the can or bag for several minutes, or by opening the container and mixing the powder well with a stainless steel mixing spoon. Then I take out the necessary amount for what I am about to do and mix in proper proportions to the corredct dilution at room temperature.
The remainder of the powder is then subdivided into relatively small amounts, placed in glass containers with good sealing tops, nitrogen is released across the top to eliminate oxygen and the keeping quality is excellent. Originally I did not use the nitrogen because I did not have access to a supply.
I do this, my students use my spoon measurements and none of us have had a problem.
As I remember, I began this procedure during the early 1950's when I had a family to rear and little money for photography so I was pinching pennis very tightly.