Yup. I think its gelling in your head! It'll sink in better in time. And your mentioned conundrum about how much ppm / overdose / underdose problems was precisely why the author of the EI method explained that in most occasions, you actually can overdose quite a bit without harmful effects. His theory allowed many to just dump and run and worry about things more important like co2 levels, keeping the tank clean, etc.
If course, hobbyists have figured out now that this is true only to a point. For example, recently someone respected did a lot of analysis and figured out that you can't really do the starting recommended levels of EI if you have hard water.
Of course, supporters of the pps dosing method will be glad to say, see, we were right all along!
They have lower starting values and eschewed the "waste" of the EI method. And they actually have a technique that relies on a tds meter to tell when you need that water change to reset your nutrient levels. So, if you have a big tank and if you're really conscientious about water consumption, this may be something to think about.
So there's a bit of nuances but overall, if you have less demanding beginner type plants like java fern, anubias, and other stuff they sell in big box pet chains, and you dont have any higher lighting, and if you are satisfied with how your plants are doing, then what you have done with flourish may be fine already.
And most importantly, what method you choose, remember, it's all about the water changing and the bit of leveling off "magic" that you've discovered if you dont want to be constantly tied to test kits after you've figured out what levels work. OR, if you're happy with running test kits for the foreseeable future, then all this mathing is totally unnecessary!
. Just test and dose or water change. It'll probably get tedious though. The nitrate test alone is a pain -- not to mention every other test. Now you see why people got creative to come up with these fertilizing regimens so they didnt have to test?
Theres another member on here who loves to remind new members to keep perspective. So, a page from that book, I'm compelled to tell you that what you have stumbled upon can be compared to a bunch of hot rodders and car tuners in the planted tank world. Just like you wouldn't start slapping new performance parts on your daily driver without realizing the implications, you really dont have to adopt any of the methods if they feel "extreme" to you. So take everything you read with a little salt and be pragmatic.
Anyway, go visit the link that I gave earlier at advancedplanted tank. It's a wealth of info and a good primer to all things planted tank so you can figure out how deep into this rabbit hole you want to get!
Oh, and welcome by the way! Post a journal of your tank, and in time, people will start joining you in your journey. You can ask even more pointed questions about your tank and members will probably be glad to help you out.
Or if not, find a journal(s) of a tank(s) you like, follow along, and in time, you'll pick up a lot as the poster shares his/her methods, pains, and successes.