Lets go through it. PPM is parts per million. 10% is close enough to your value. Lets look at a gallon of water, thats 128 ounces if its an american gallon, so 1 ounce puts you at (1/(128x20gal))x0.1 or about 391 ppm but that's too much so try 1 tsp which is 1/6 of an ounce and that gives me about 65 ppm. Since I only want 1/10 of that, I start with 1/2 a ml which is 1/10 of the 5 ml equivalent of a teaspoon or about 1/2 ml. To be on the safe side, try half that much and make sure your math has not thrown you off somewhere. When you test, if you get 3 ppm you know your math was right and add the other half of what you wanted. Otherwise, you are close and can adjust.