best way for you to find out is to take out the rubber stopper on the cartridge end. While you have it out. Start dropping from the battery end. Keep dropping till the other end of the the cartomizer ( the cart end ) starts pooling with ejuice.
That's what I did with a friends cartomizer, he wasn't into taking out the white rubber on the other end of the cart.
Or better yet just take out the white rubber end so your not guessing how much you need.