The carto that took 40 drops was washed out and bone dry. If the carto is not fully empty it would of course take less
juice to fill it. I expect that as the batting gets old it shrinks down and will not absorb as much either.
I stop filling when the carto looks lightly slushy on top. Give it a spin and if it looks dry again I do add more, try fewer drops at once if you are overfilling. I stop when I can still see evidence of
juice after a spin, if it's not full it looks quite dry, when it is full you can see
juice but the top won't look soaked.
Make sure you have the battery end of the carto at the bottom of the bag, hate to force the juice to the top instead!