Since you said you are using the kr808 don't listen to people talking about taking stuffing out or dripping.
The kr8 is a cartomizer design and works very well. A good cartomizer will last a bit over half a pack of cigs, or maybe a bit more. Your usage of 25 carts to replace about 10-15 packs would be about the same as I found.
For refilling you have 2 really good options. If your carts have a hard plastic end on the mouthpiece end, then the end is glued on. With these its easiest to use the condom fill method. If the end is a softer rubber, then its easy to pull it out and fill by simply dripping new juice right in there.
In the case of the condom method. Fill using the condom till juice starts trying to come out the other end. Then put the cap with the plug back on the battery end of the cart and put the cart in something where you can spin it around to force all he juice toward the battery end. Then fill it even more and do that again until its plenty full.
To know if its completely full blow gently through the cart. If it gurgles its full. Get a paper towel and blow just a bit out just enough that it stops gurgling.
A normal cart can hold about 35 drops of juice and the newer carts can hold 50 if you fill them this way.
Thanks much for your time and effort.