I just tested the common plastic dropper bottle that most
juice comes in. 20 drops = 1.05 mL. Ok, .05 mL is hard to read so I did it with 40 drops and got 2.1 mL, and again with 80 drops which came out to 4.2 mL exactly. I did the same test with a another random plastic dropper bottle from my collection and it came out exactly the same.
Conclusion: If all you have is a (washed and empty) plastic squeeze dropper bottle, 20 drops = 1 mL+ an almost insignificant extra volume (.05 mL), i.e., close enough.