The first is a guide All you need to know about referrals
Interpretation of Laws
When using technology to determine where the ball pitched (as per Law 36.1(b)), the TV umpire should refer to the “point” (or centre) of the ball. Therefore if at least 50 per cent of the ball pitches outside the line of leg stump, then no lbw dismissal is possible.
When using technology to determine the point of impact (as per Law 36.1 (d)), if ANY part of the ball is intercepted between wicket and wicket, then the point of impact is deemed to have been between wicket and wicket.
When using a replay to determine the moment at which the wicket has been put down (as per Law 28.1), the TV umpire should deem this to be the first frame in which one of the bails is shown (or can be deduced) to have lost ALL contact with the top of the stumps.
It is daft to use the "point" (centre) of the ball to determine where the ball pitches. If the pad is not involved the ball doesn't need to hit the stumps full on: as long as it clips one enough to dislodge a bail the batter will be out.The second Cricket referrals: the great debate continues... includes a link to a video of the Jacques Kallis lbw which was overruled (or whatever the official term is) by the TV umpire. Worth a look if you've not seen it before.