Thanks Hefe and Mark. Great advise on making a stock and dividing it up.
I've got a set of pretty accurate scales to 0.01g, but at 16.4mg it would be exactly that, 0.01g or slightly more. In these dimensions I don't really trust the scale as the slightest breath will trigger it.
So will be making a stock with distilled water and use it tomorrow. Also useful for the heap of other substances I ordered and only need similar quantities at a time.
Edit: And yes, the Zink Sulfate Monohydrate is very water soluble, unlike some of the others.