@Myfriendis410 is correct, you can’t realy pH distilled, demineralized, or otherwise “pure” or zero ec/ppm water. Or maybe I should say you don’t need to, this level of pH accuracy is not needed for home hydroponics.
The pH scale stands for “potential hydrogen” and is a scale of acidity and alkalinity, with 7.0 being neutral and above 7 is alkali or “basic”, and below 7 is acidic.
If water doesn’t have any dissolved minerals for hydrogen ions and anions to create acids or basic compounds, like purified water is lacking, then it is generally going to be neutral in pH, or pH 7.0 and so you don’t really need to measure the pH of the water until after you mix your nutrients and let it stabilize.
Demineralized or purified water can show a acidic reading from absorbing CO2 from the atmosphere, but this is a very weak acid, and it will not really “hold” or alter the pH of anything it is added to as it is such a weak acid when formed from regular exposure to air in this way. And again the reason you don’t really need to measure the pH of this very pure water.
Add your calcium and magnesium, mix very well, and the let sit for a little bit, maybe 10-15 minutes, for everything to stabilize, then test pH, adjust pH and mix and let stabilize, and repeat, until you reach your desired pH, 5.8 for hydro and 6.5 for soil.