at issue is what can be measured quantitatively (but expensively/tedious at least)... and what can be estimated with practical means by average hobbyists.
Most folks fall into the latter category.
So when you start adding say .5 oz of home-grown phyto to your system per 100 gall, and notice little or no increase in... say, skimmate production, but then ramp up to adding 2 oz of phyto per 100 gallons of water and notice a distinct increase in skimmate production - its gotta make you wonder
In kind... if the addition of phyto seemingly doubled the growth of your cluster fanworm population (ascidians, poriferans... whatever)... but the tripling of yor phyto dose did not further multiply the population, you have to wonder again.
Another poor man's test to get you in the ballpark: test your DOC levels at the start and track them over time. Were they the same or similar in the weeks prior to light phyto feeding but steadily climbed as the use/abuse of phyto by volume also increased?
This might be telling about the phyto that was excess but did not get skimmed out obviously, but dissolved instead.
You will also notice a steady fall/flattening of RedOX values from a persistent overfeeding of most anything. This indirectly reflects food/nutrients that are not banked (used by living organisms) or exported (ala skimmer for example), but instead have dissolved and become a burden to water quality.
Indeed... these are not scientifically exact by any measure... but then again, they are part of the reality that we can decipher in trends of water quality.
In my opinion, phyto use is very helpful; in small regular (I do daily) quantities. But it is commonly overused in aquaria that are dominated by zooplankton feeding corals.