As BenGman told you you always water with nutrients. This isn't as complex as you're trying to make it.
If you want to set up a complex water water ca/mg water water nutrient several times a day you can do anything you want. If your plants show deficits or toxicity just change it around. But I'm far too lazy. If my plants need watered they get it from my mixed res with the nutrients they get every day.
The only exception is once every couple weeks I'm too hot, lazy or doing something else so I water my plants with tap water only (mine is around 250 PPM) this is flushing in hydro so I leach out any excess salts that might be left in my medium. Then I grumble and mix another reservoir.