Well, I only fast for three days... during which I have no food. This means I need to get ALL my electrolytes from it, but means I only need it about once a month.
I usually end up weighing the stuff out for 1.5 days, by using multiple plastic shakers. Each shaker has dry ingredients. Twice a day, I fill one up with water and then guzzle it. After a day and a half, repeat. After three days, I'm back on foods.
I used to play with making one big container of minerals mixed with water, and just adding the fiber as I drink it, but some minerals don't dissolve in water... they really need stomach acids and other digestive processes. And that sludge would settle. So now I just mix per batch.
I think you can do ok making a dry pre-mix and learning to dose it out per day, or half day... if you make enough for five days, and it weighs, say, 40 grams (random number), you can weight out doses of 4 grams each, twice a day, for five days - ten doses total. I find that if you put the blended powder in a shakable container you can easily re-mix it, and settling doesn't appear to be an issue... if the mix is slightly rich on something in day one, it'll be compensated on the last day.. but it appears to be pretty much the same.
I used to worry a lot about settling in my soylent DIY, but I've found that concern to be unfounded, and the same is true for the electrolyte mix. The only real gotcha is making it too dense with fiber and then not enough water and letting it gel up. Ugh.