Check my emphasis added to your message.

This is exactly where your focus should be. That means that you are seeing the battter's feet with your periphial vision (the corner of your eye). You better be d*a*m*n sure that the batter's foot is on the ground outside of the box prior to contact.

I do recall a HS game a few years ago where I called one of the slappers for being out of the box. She was facing a pitcher that was not good and had very little speed. The slapper would start her run in the box, but because the ball was coming so slow, it threw her timing off and she ran literally two steps in front of the box before she hit the ball. Easy call.

The slap hitters are a challenge in fastpitch. I did notice the gal that stepped in the front of the plate. Again this is shown through the angle of the camera from behind the pitcher, so it is tuff to be certain.

The only way for this to change is to change the rule. Both feet entirely inside the batters box (meaning no part of the foot outside the line but could still touch the line) and then the batting will be fairer. Still a judgement call and could be easily missed or argued.

It doesn't make any difference what the rule is or how it is applied, as long as it is done equally across the field, no one has an advantage, thus it is as fair as it can possibly be.