Now folks, EI is simply a general thing, a starting point, it's not written in stone, nor are the suggestions I make about it.
Don't fall into that trap and limit yourself.
EI can be easily changed without a test kit also, you simply slowly reduce the nutrient of interest, in this case PO4.
It's not hard, nor is there any need for a test kit.
You will run into to trouble the lower the ppm 's you go with any test kit.
Accepting that assumption will help a great deal.
Next, simply maintain the higher levels and lower the nutrient down till you get a negative plant or algae response.
You need to maintain good non limiting conditions for the other nutrients when you do this and it takes about 2-3 weeks to get a good feel for how the limitation works.
It does not have to be PO4 either, CO2 works better if you ask me and achieves the gaols of less work... better than PO4 limiting.
But the trade offs each limitation provides is what you will want to focus on.
If you run the PO4 down low, the NO3 does not need to be as high.........but.........if it is high, it's not an issue if you do the water changes either.
But KNO3 is roughly 22$ for 50lbs, I really could not give a hoot if I wasted 2x as much at those cost for my 50 gal tank.
But either way is fine(high or low NO3 if you limit say CO2/PO4).
Simply reducing the amounts dosed progressively and slowly till you hit a negative response is all one needs to do if they prefer to try a lean version.
Amano's tanks are not quite as lean as he may lead you to believe either.........I know folks write things about their parameters and they fudge them.
They also .......as everyone here knows............change day to day and hour to hour depending on the nutrient in question, Amano is not providing standard error and variation over a week, over any time frame.
Water column nutrients also will tell little if you have a fair amount of nutrients in the substrate, if you run things leaner in the water column.
If you seek to slow down growth, stop adding so much light.
"I want slow growth and healthy fast growing stem plants but I want 5 w/gal"
Use things/methods that do not oppose and are not anatagonistic towards one another.
Use less light first, then CO2, then PO4.
A smart aquarist will simply add a nice scape with wood/rocks etc to replace all the faster growing stem plants or switch from fast growers to more electic slower growers and Crypts etc so they prune less.
Why keep weeds and expect them to grow slow?
I'm not sure why folks paint themselves into a corner and assume such rigid approaches, EI is a very flexible methiod and allows anyone to modify and change things simply by reducing or increasing the volumes of ferts dosed.
This is much easier than testing and assuming the test kits are correct/calibrated correctly, the volume of KNO3 per unit tank volume over time is a much simpler thing to suggest and to standardize for folks when giving advice.
The question is what is gained by lowering the nutrients, the answer is slowing the plant growth without getting algae.
Non CO2 tanks beat any method for that one.
Now you still want to add CO2, well....best option is to reduce the fast weedy species.
There are many goals and constraints we place on a method
Being able to achieve the goals you want with a method is another matter, 99% of the issue is you, not the method itself.
If you can do each method successfully, then you can better judge and use such methods to achieve various goals you may have.