I'm a soil guy, but been slowly looking at hydro. (probably won't work for me)
But I just read recently, you want the ppm to remain constant.
If the ppm is dropping then the plants are using more nutes then water.
If it the ppm is raising then they are using more water then nutes.
And to forget about 'mfg's target ppm' and find the equilibrium that the plants want.
I'll let the hydro guys beat me up now.