I have been mixing, testing and using developers for over 60 years and I have found, by doing it professionally, that you must be conscious of the potential chemistry going on, and you must devise a standard process test and a standard keeping test. The process test must consider capacity and change per unit usage. The keeping test must allow for the concentrate and mixed working solution. It must consider factors such as how much your first print is equal to your last print (seasoning).

In the end, it means that your first print from a mix of developer should equal your last print, that a lot of prints should be able to be run, and the time span should be maximized.

This is a long and lengthy process. It is also, to me, fun.