Gee, and I thought I was being excessive when I put about 2/3 of a dropper into a new dry unit! 
For refills, I use about 1/3 of a dropper. My measurements were made after running one on the bench and testing how much it took to fill and overfill a smoke unit.
There are smaller smoke units in some locomotives, they require less. The little MTH diesels come to mind with the single element smoke unit, it's pretty small. You have to be more careful with those.
When I replace the wick, I saturate it before final assembly so I can control how much is in there.
