I have only done a couple of blanks. And dropper bottles can have wildly different size drops.
This guy is filling an older Vapor4Life carto I think since his soft cap seems harder and his stuffing may be a bit looser. But he is putting in over 20 drops.
(note - an unbent paper clip works fine for popping out soft-caps. This video includes straightening the center channel which is only needed if you can't see light
thru the carto)
YouTube - How-to guide to refilling KR808D-1 or Vapor King cartomizers
I don't worry about over filling - no matter how careful you are some may drip out the threaded end. I hold a tissue on the threaded end. Some leave the threaded end V4L hole plug and condom on. When the juice doesn't want to soak in any more, I clean it up on the threaded end - yes a bit might drip out. I puff into the threaded end with the open end down into a tissue to clear the air channel. I use the folded tip of a tissue to clean up any juice pooled above the stuffing. (instead of all that, some stand the carto vertically, threaded end down on a piece of tissue to let extra drip out). On a fresh fill you need to let it sit a bit (I lay it on its side various ways) to let the juice soak into all of the dry stuffing so it will be wet near wherever the atty is.
When you first start
vaping it, if you cleaned it up well, it should not sputter too badly. It may be a bit flooded to start. That's better than singeing dry stuffing. Watch out for hot sputters and soon it will start vaping fine.
I add more juice whenever the vapor seems light or a bit dry and holding the ecig carto down for 5 seconds doesn't fix it.