Well, that is true and if that is what you meant, good on you.
What I have found is pH is meaningless in practice since it drifts by a whole point or more in 24 hours, responding to the light.
But, if I don't measure PPM I have no idea.
So, for pH, and again why I don't change feed, is a set ratio of feed in RO water will only have 1 pH, no matter the concentration, the water being almost neutral.
So, I set initial pH, after taking notes and measuring etc, simply by measuring in the feed and supplements. In fact the ratio of Ca/Mag/Si, I use is around 5.5 and with the water, and feed I come out at 6.1. Done.