Go to your nearest perfectly drawn field, and put your foot COMPLETELY out of the box. Look how far out of the box your foot is. Touching the plate or in front of the plate, right?
Now use that as your barometer going forward. MOST of the time, even when it looks like they are out of the box, they aren't.
(Slap hitters are a different animal... and even HARDER to call, even WITH lines!)