I add prime or Amquel based on how much tap I add back.
I squirt and refill the tank back up.
Same temp(your hand is accurate to about +/- 1 degree F difference- pretty good really).
I'll add the ferts right after once the tank is full. Traces, KNO3, KH2PO4, GH booster. That's it.
You do this every day at 50%, you do not need to any other dosing and the system is extremely stable, but you cannot over do the water changes nor is it difficult to reset any dosing errors you might make by simply doing an extra water change or if there's a problem, do another water change, or a larger % say, 70% etc.
This resets the tank and rules out any nutrient related issue, because 70% or more of the tank is now just like a reference standard solution, that is the idea behind EI to begin with, 50% weekly seems like a lot of water to change unless you have high bioloads, but it really does not take long vs say 20% and we have clean and prune etc more often than that generally.
If you do daily 50-70% and dose back right after the water change, now after a week or so, you are going to be very very close the reference solution, no test kit needed.
So you can rule out any such errors with nutrients rather easily with water changes and dosing with dry powders or liquids. Light can be measured some what reasonably these days using a PAR meter, leaving only CO2 as the thorn in everyone's side.
Common aquarium care things like filter cleaning and other basics often get over looked at times also. But those can be addressed also.
The general idea it limit the amount of variables, rule things out and leave yourself with only a few things to monkey with. If you try and measure and test every darn thing, it's simply too much work, data to mull through, too hard to interpt.
Some think they have to do that. There are easier ways to get around it.
Some might hate water changes, so they are willing to do the calibrations and use the test kits and live with the assumptions. That's fine as well. I have great tap, so I use it and make and automate the water changes to remove the issues with changing water. You cannot automate the test however. I also have my arm in 24" or more of water if the tank is full, I like 1/2 the water gone so I can work and prune and tend the tank, this hobby is about aquatic horticulture, not set and never touch again.
You can do either, but generally it's better for most folks to get in and prune and clean with the tank 1/2 full, so while it drains and refills, that gives you about 10-60 minutes to clean inside depending on the fill rate etc.
Regards,
Tom Barr