Put another way, if you dose CO2 and light, fish etc, but do not add anything, then suddenly add ferts, well, yea, even if the amount is limiting, adding ANY ferts will yield more growth/better planted tank.
The upper bound of dosing(non limiting) is a more useful tool in Science and for the hobbyists.
This way the ferts are now
independent of other factors like CO2 or light. So that dosing routine will give you a fert method that target any and every panted tank.
This way, you can focus solely on CO2(or adjust lighting) as the dependent variables.
Now, dosing non limiting ferts can be modified and slow and progressively reduced and with good observations, able to hit what is called the critical point(90%-100% growth rate). You bump the dosing back up to the last prior amount, that is your target specific to your tank.
Then you can reduce water changes and not need to test or worry about ferts/CO2/light, you can focus on the fish and the gardening. But if you keep larding it on, there's no risk as long as you do good sized water changes, which as a discus owner, you likely do 1-2x a week anyway.
EI was suggested for this reason and it's simpler than PMDD(which is really the real method you suggest, they(PPS) seem to have plagiarized 95%). Douche baggery.
EI is hardly new either, most of it came from PMDD, in fact, 95% did. I give credit where it is due. PO4 dosing is mostly my own suggestion, so that credit is my own, but even there, not really, I just dosed it unknowingly. Another person, Steve Dixon pointed it out and tested it, then I followed it up.
This happened back in the mid 1990's.
Estimative Index
As there have been many articles written about EI and in many languages now, and it's been around since about 1996.................obviously I cannot control myths and other added content to the methods, many have a very rigid mindset about it.
That is unfortunate.
But we can try and correct folks, offer some history behind it and better yet, offer then a simple tool to make ferts indpendent and then you can focus more on the skills you originally wanted when you started this hobby: gardening a nice planted tanks, not fiddling with test kits, dealing with indirect limitations and algae.
Ah, but what do I know about Discus and planted tanks? Ask my Clients: