Have a look at these videos from Laidoffsick:
https://www.youtube.com/watch?v=WRcpFRtA6IY
https://www.youtube.com/watch?v=1asUWbJtfok
I am experimenting with 1 drop of fluid per 1 minute of run time and refilling after 10-15 minutes of run time; therefore, 10-15 drops from a Lionel graduated smoke dropper. I don't know if that is effective since I have not taken a smoke unit apart for some time. My objective is to not burn the wick.
A 2014 VL Lionel Big Boy was my 3rd Lionel engine; I toasted the 3 smoke units in it by starving them of fluid along with the RCMC; learned the hard way on how to care for a smoke unit; at that time I was gauging refills by watching for thin output; that didn't work for me.