I found it to be quite a pain to refill cartomizers and some of the newer cartomizers require way too many drops. Also I am quite of a spaz and have had drops all over the place. What I do now is take one of the end caps on the cartomizer and fill it about half full of juice. I then take an empty or blank cartomizer and press the threaded end slowly into the the filled end cap. No drops required, the correct amount of juice is automatically put into the cartomizer, you do not have remove the end cap on the cartomizer. You will know how good a used cartomizer is from the amount juice it will take to fill it.
Maybe this is in use by all you guys and I am an idiot to suggest this but if you like this and have not used it in the past give me a reply.