In or out of batter's box

[CecilOne]

I know we just discussed this recently, but I can't find the thread or the specifics in the book.
For the case of a batter hit by a batted ball, the result is different if it is fair or foul. We rule it foul if the batter is still in the box. If I have learned it correctly, the batter is considered in the box if either foot is still touching in the box. That would apply even if the part of the batter's body that is hit is over fair ground or one foot has stepped out after batting the ball.
Q1. Is that correct?
Q2. Is it specified anywhere in any SOFTBALL book (esp. NCAA, NFHS, ASA, PONY) or is it just the accepted intepretation of "in the box"?
Q3. Do you know which thread our previous discussion was in?