Yeah, the foot has to be over the chalk completely to my understanding. And I also agree that it is hard to ask the home plate umpire see the batter out of the box on the swing. The other umpire in the field can watch this as well though, but might not be close enough to see for sure.
My main concern is the ball hitting the batter while he is out of the box, which should be seen by both umpires. But, like I said earlier, if the box isnt there, you cant call anything as an umpire.