I guess you slowly cut back on your dosing to observe when you noticed a change in plant health? If so was that based mostly on growth?
For KNO3 it's based on testing of Nitrate level over a time. For example, the one tank that I don't dose any KNO3, the fish/fish food, etc produce enough nitrates that I HAVE
to do a water change every week so it doesn't need KNO3 added. The one tank I dose full EI dose doesn't have any fish ... just snails & plants with a bit higher light than my other tanks. So that tank needs a fair amount of KNO3 added, otherwise the plants use it all up. I only change the water every other month or so now. It's maintaining about 10ppm nitrates which implies it's not building up too much and the plants are using we they need. After thinking about it, it's probably not full EI dose anymore. I may have started full dose & adjusted down because the nitrates were building up to much. And also extended the water change schedule.
The other ferts have been trial/error based on Phosphate tests, observation of plants, tank (if any algae), light & water change schedule. For example one tank I only do water changes every 2 weeks, it's a heated tank and it's low end of medium light so I may have started with 75% EI dose 3x week. Then adjusted down to 50% or maybe even 40% IE dose. I use this fert calculator: http://calc.petalphile.com/
to compare regular EI vs EI low light weekly dose, target levels and then make guesstimated % adjustments based on my water change schedule, fish load, tests, etc. By not doing weekly water changes I've complicated my fert dosing but over time as I get it dialed in it will save in the long run ... not only time but water. I do try to conserve water the best I can as I don't believe it's really an unlimited supply.
I did a lot of testing at the beginning trying to get the dose right but now mostly just observe the tank/plants. So far it appears to be working as I don't have algae problems (except my 75gal cold water tank that I just can't seem to totally get rid of the brown diatoms .. been setup almost 1 yr) and plants are doing as expected. I say as expected because 2 of my tanks are cold water tanks that run about 58-60 degrees in winter so I don't expect a lot of growth for plants in those tanks. Also, I'm not trying for fastest growth possible. My objective is a tank with no algae with slow to moderate plant growth and healthy looking plants.