for me 1 drop is about equal to one nice long puff. sometimes 1.5 or even 2, depending on how large of a draw i want. 1 drop per draw is usually what i take.
this is using a standard dripping atomizer. joye and empire mods are my go-to attys. but the same holds true for my 306 atty. keep in mind that on a new atty or a freshly cleaned out one, the first few drops get soaked right into the mesh and is virtually unvapable at first. so youll need closer to 6 drops for a few vapes for the first load or two.
using a cartomizer, the equation will be different due to how cartomizers wick juice differently. you'd probably get more draws per drop with the filler material method that cartomizers use. most of the "wicking" on atomizers is done instantly, as the jucie makes direct contact with the coil. but in a carto, you rely on the filler to transport the juice to the coil, so you can only vape as much juice as the filler can transfer during your draw.
regarding your carto turned dripper thought, youll likely get leaks with anyhting over a couple of drops. removing the filler and exposing the coil does work to atomize juice, but the carto has nowhere to hold the juice you drip in, and most of it will likely come flooding out the bottom before you can really get a grasp on the flavor. ive only tried this with one brand of cartomizer and i hated the results, but YMMV.
hope that helps.