Don't fret about testing. The test kits for the stuff we are dosing are next to useless in terms of accuracy.
One of the main principles of EI is that in 99% of cases we massively overdose what plants could possibly need. Followed by a large weekly water change to reset the water column (the idea being that we don't want ferts to reach crazy levels as unused ferts roll over from week to week).
There are some caveats to this that might require you to make some tweaks. I.e - I live in an extremely soft water area, my water contains next to nothing. So I have to supplement my water with extra calcium, magnesium and iron.