When is a Batter out of the Box?

[Manny A]

When a batter is contacted by his/her own batted ball, by most rule sets he/she is out if the ball is in fair territory. But the prevailing interpretation is that he/she is given the benefit of the doubt while still in the batter's box, and the call is a dead ball followed by a foul ball ruling.

So how do we define when the batter is out of the box? Is it just one foot completely on the ground outside the box, similar to the rule on hitting a pitch while out of the box? Or is it both feet?

I had a play this weekend where the right-handed batter bunted the ball just in front of home plate, and as she took off for first base, she made contact with the ball with her front knee. When it happened, it appeared her front foot was on the ground in front of the plate, but most of her body, to include her back foot, was still in the batter's box. Basically, she was more inside the box than out, but her front foot looked to be outside and on the ground.

I've always called it where it's blatantly obvious the batter is well outside the box, but it's that grey area that I'm not too sure about. How is it defined?