Did you buy calibration liquid too? Cuz that is pretty important using digis. It may be good practice just to get the drops and use both until you get comfortable with the amounts of drops you'll be using to get your solution in range. I've found that for exactly one gallon of water with nutrients already mixed exactly 1ml of ph down will drop my solution to the 5.9 neighborhood. Also 1ml = 20 drops! Hope this helps!