Indeed, 15-20 drops is about the amount it takes to refill a used cart without wasting juice. I use a syringe to inject the juice in from either the bottom of the GreenCig carts that I use or from the other end, you just pop of the endcap, remove the o-ring, and there is a small hole on the inner cart that the factory used to fill the cart and I pop the syringe in there and fill it up just like they do at the factory. One thing I find to be useful is to lightly blow in the end of the cart to test for excess juice while refilling if am just filling it from the atomizer end. If no juice comes out I continue to fill, and if it does I will give it a quick spin and see if that helps and add a little more and repeat the light blow out of the end to make sure it's all good. Having never filled a blank cart I have no idea how many it would take to fill it.