confused?

[cocobeware]

I always feel unsure as to how far across my jab should come so my shoulder protects my chin,Should it be a case of my shoulder coming across to protect my chin or tucking my chin in behind my shoulder?
Maybee i could explain myself better if i put it like this...If i stand parallel feet shoulder withdth apart and just punch straight out in front,I think this is like they do in karate,Is it the same thing when im trying to achieve in my boxing stance with my straight jab and right?