20 tsb is about as much KNO3 that will dissolve in 500 ml of water. 17 should be doable, but it also might get tricky when you add the P and possibly extra K in the mix
Im wondering how many ppm of NO3 are you trying to hit with each dose? A standard EI dose of 7.5 ppm, 500 ml of water, 5 ml dose, calls for roughly 13 tsp.
So it sounds like you are either trying to dose more than 7.5 ppm, which is fine, I do it myself, or you'd like to use extremely small doses.
Use the calculator linked above in d33pVI's post. And as already mentioned, the simplest thing would be to go with larger doses, 10 ml instead of 5, etc
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