Answer:
there isn't a distinct measurement to how deep the needles should pass into the skin, but you should be working from the tip of needles, if your not deep enough, after wiping it will look like you have only scratched the skin with gray ink, you need to go a tiny bit deeper. the setup of your needles in the machine might make it easier. with the armature bar in its resting position only the very point of the needle should be visible, were talking the tiniest glint. when you push the armature bar down to protrude the needle it should move out about 3cm of needle, the machine tip should not be near touching the skin.
that's the best i can explain in words, practise makes perfect though.