Just bought a bunch of Lionel Premium Smoke fluid in Pine, Sugar Cookie, Chocolate, and Peppermint - all smell great but haven't used any yet. Pine is the strongest and my favorite. The dropper is graduated in .25ml increments. I counted 10 drops from .25ml and 20 drops from .50 ml. give or take a few drops.
To further add to the issue of Lionel smoke units, you may want to look at
https://www.youtube.com/watch?v=WRcpFRtA6IY
However, I am no authority on the care and feeding of a Lionel smoke unit - 3 have failed on me so far or I have either starved them or over fed them.