In answer to your first comment Alan, as you know, powder is classified into types and those types require specific settings on the powder dispenser that is commensurate with the powder size. Also, powder is made in uniform pellets or flakes (photo chemicals are not). Powders are also generally of the same or nearly the same density. Therefore, in the case of powder, you have what is effectively a calibrated variable sized spoon. Also, when rotating the dispenser, you 'cut' individual powder particles to make an even 'spoonfull'. Therefore, the argument becomes unimportant in this case as you cannot (or do not) cut up individual crystals of photographic chemicals to insure a uniform volumetric measurment, but you do in reloading. These subtle details are often missed.
BTW, I reload .223, .243, .45, .9mm and 12 guage. I encounter this quite a bit and have had a lot of time to consider the problem. I use a variety of powders and have verified the accuracy due mainly to uniformity and grain cutting! This differentiates it from the photographic use we are discussing here.
As to your second post, the cation has several functions in increasing fixation or retarding it. One is size which relates to diffusion. Potassium ion is larger than Sodium is larger than Lithium, but Lithium is about the same size as Ammonium. Therefore, where or how does the fact that Ammonium based fixers are faster than Lithium fixers come in?
Well, Ammonium ion is both a cation and a ligand and therefore the size of the Silver Ammonium Thiosulfate complex is reduced considerably by the fact that the ammonium ion becomes part of the complexation cage with the silver ion. It is not an external cation. This tends to speed the rate of both soluabilization and diffusion. The rate of dissolution of silver halide is due to the effective syngergy between ammonium ion and thiosulfate ion.
So, as a matter of actual practice, it is a fact that there is a syngergistic effect going on here. This was not fully understood until work in the 60s and 70s which I took part in. Mixtures of silver halide solvents become synergistic and speed fixation.
See my Super Fix formula posted here for an example. Jay DeFehr posted it from the thread on Photo Net. The entire thing played out there.
Hope this helps.