The 33% add back method or "rule" is a general starting point that covers the bulk of strains out there. It is effective but, if you see an issue...you adjust... The 100% solution. Then the adjustment carries over to the add back solution..
BUT
Yes you do some adjusting for strain needs. Like Mg hog's and such.
Mostly for advanced growers that are "pushing" or specific result supplementing. That generally needs some basic nute adjustment....
In volumetric feeding in hydro... I generally measured my nutrients by weight....
I suppose that makes a little more sense, though I still don't have the full grasp.
I was taught that when ppm drops, you top off with the same feed you out in, based upon the ph swing.
When ppm rises, you top off with water. Principle being, you add salts and salt doesn't evaporate, only water does, so concentration of salt/waste is higher. This also shows in ph.
When ppm is unaffected, time to rinse, clean and refill because it's mostly waste or on the verge of a salt buildup, though buildup is dependent upon nutes used. Hence, the residue in the bottom of the res and even sometimes on the roots. Ph is usually unaffected as well at the onset.
I usually rinse, clean and refill when I notice ppm starting to drop less and less.
I have never encountered an instance where ppm going higher than what you started out with to be a good thing. Perhaps that's just the OCD of something is not right for me. Perhaps that would be fine. If ppm goes higher than what I start with I'm still rinsing, cleaning and refilling, whether good or bad. I consider the roots to be sitting in their own piss and shit at that point.
Also, never had a root rot issue that way... Ever.
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