Gerald: the paranoia - I'd agree on that one. Now, I wonder how unhealthy tap water is, taking into account old piping systems... OK, thanks for the encouraging words. I'll keep hunting.
PE: Thanks for the details, I had read similar posts of your's already. It would be great if you could specify which components need the accuracy, in particular which one needs the 1mg accuracy. I trust I can achive that accuracy in any case, but don't want to put in an effort where not needed - avoiding paranoia.
While in general I would agree that accuracy is needed, I should state again that my main intention for mixing chemistry is curiosity as well as taking personal control and not a hypothetical _technical_ ideal. And in that sense I am perfectly happy to 'eyeball' my results but not only with a quick glance also with a magnifier, i.e. on the micro level.
Whether, say in a portrait, the skin is a bit more warmer (yellow) or cooler (blue/olive cast) is not only a technical question but also an aesthetic one. Grainy (B&W and C) shots can look great or ugly. Look at lots of the fashion/lifestyle stuff - how off are colours there. Product photography uses lots of subtle lighting to create well pleasing images. Compare an image of a Big Brand Burger to the real thing.
So, for me, if the results of any homebrew are optically pleasing on macro and micro level, I am happy. If not, I'll move on. Experience gained, lots learned. Just like I learned that most labs with professional control just don't give good results. But on them I have no influence. At home I have an influence. Even if not technically perfect, that is the kind of control I like to have.
Guess I am saying the end justifies the means as well that I'm in it for the journey. But that only has validity for myself