Hey Buck! Long time to chat. It's good to be back reading all the great stuff posted here.
As for your question... You're off a couple of decimal places. A gallon is 3790 ml (3785.4 if you're picky
). It would take 70 tablespoons of KNO3 dissolved in 1 gallon of water to have each ml add 1 ppm to a 50 gallon tank. I don't think 70 tablespoons will dissolve.
If you did 7 tablespoons in a gallon, then each 10 ml would add 1 ppm to a 50 gallon tank.
Also... just a trick... If you set the "Amount of water to mix with" number to 1 ml, you can see how much chemical you would need to add directly to your tank to raise the concentration by "x" ppm.
For example, say your nitrates tested at 0 ppm and you wanted to immediately get the concentration up to 5 ppm in the tank. Set the tank size to your tank size, set the "amount to mix with" to 1 ml, then adjust the grams/teaspoons/tablespoons amount until you get an "Each ml will add..." answer of about 5 ppm. Then mix that amount of KNO3 in a small amount of water (just enough to get it to dissolve) and pour the whole thing in your tank and you've raised the tank concentration by almost exactly 5 ppm. In your case, it would be just over 1/4 of a teaspoon of KNO3.
Probably more than you wanted to know, but I've been away and I'm really glad to be back!