The take-over line should be the same for a Check Captain or Line Captain. Line training, is not learning how to fly, it's type and SOP's familiarization. The point at which control should be taken should be the same line regardless, and that is as soon as safety of flight becomes an issue. If the approach looks like it will end badly, make sure all the correct calls and procedures are followed, as expected of a normal line flight. If you want to see how far a candidate will go wrong, stick em in the sim again and play games, not in the aircraft.