Here is a great short tutorial about mixing nutrients. He uses the recommended concentrations of a specific brand for hydro, but in general and regardless of nutrient brand and the mixture ratio/concentrations may be different but otherwise there is some good advice that should be used whether mixing your nutrients before pouring them into your hydro reservoir or pouring them on soil or over your soil-less media.
I’m thinking waiting as long as he did between adding new nutrients was a bit excessive, the main thing being you mixed it in the larger body of water and thoroughly mixed it before adding the next one. This goes for adjusting ph as well. I also thought the fresh lemon squeeze to adjust the de-chlorinated tap water to below 7.0 was kinda neat. Pure water is almost always 7.0 and purified water and most bottled water are always pretty close to neutral in pH, however some bottled water and tap water is often a little on the alkali side, 7.5-8.5. Too acidic of water is not good for plumbing, and too alkali of water is often a result of “hard water” and results in too much mineral deposits in your plumbing fixtures.