[Art N]

For this type of BC violation, where FC status was established by A, I think it may help you if you consider the division line like an end line or side line. If A touches any of those lines (with the ball, their feet,...) then he/she has violated.

This also helps to make the call when B bats a ball from A and it deflects off A's body...goes into the the BC and A is the first to touch it, then you have a BC violation. If a batted ball went off A's leg and went OOB, you would award it to B. Not exactly the same, I know, but I think you get the idea.