Way back when (even predating the "Barr Report" ..... who else remembers TB hanging about on the APD mailing list, asking loads of questions, then contributing to discussions, then dominating discussions, then finally setting up his own "pay up front" website
😉 ) actual.weights of the compounds were used such that concentrations of stock solutions were known - so if you wanted 5ppm X in the water column, it was easy to calculate the volume of compound X stock solution needed.
Then one would/could check the level of X in the aquarium using various test kits - with a strong preference for certain branded test kits which included reference standards so that one could check accuracy/precision of measurements.
Of course actively growing plants will consume X, so testing the level of X in the water column, just after addition & then over time, was done so as to have some idea of what to add when to a given tank.
I was running a million phosphate (& other) assays back then & sceptical of the quality of aquarium trade kits & had access to analytical equipment & various reagents, so amused myself with standard curves & such .... & found those test kits weren't half bad - not 3.62 ppm nitrate accurate, but 5, 10, 20 ppm, & dilute your 20ppm sample by a factor of 4 & measure 5ppm well enough
Test strip reagent pad chemistry also yields pretty decent data - IF handled & stored appropriately (& one got the brand right)
I read a lot of anti-kit dogma on forums, but very few details of how these kits "failed"
😕
or what the supposed interfering compounds are

( & there's an amusing faith in pH values even when the measuring probes have had no maintenance or calibrations done
🙂 )
I don't test anymore - but that's just because I'm lazy
😳 - I also just grab a bottle of Tropica's finest when I want to "feed" my plants
I tend towards sensitive fish (mostly wild caught) & (in)bred pretty shrimp lines, so I run my tanks lean with moderate CO2 & lighting
I do occasionally dip a strip reagent stick just to confirm that my tap water is still pretty much the same as my tank water