At what point should an umpire raise the finger for an LBW decision?

[happydavy]

It's only a matter of time before it happens in some important match and become a matter of debate - when should umpire raise the finger for lbw decision - immediately as the bowler appeals or wait till the ball becomes a dead ball so that if his decision gets challenged and overturned then the runs scored by batsmen ,while running, be counted. Today against Zimbabwe, Bdesh lost a wicket on the final ball when umpire adjudged lbw, which in this case was the right decision though challenged by the batsman. However, even if batsman would have won the challenge, Bdesh scored would have remained the same, which i think is unfair, as batsman had a lot of time to run a single or even a double. A team should not be penalised for umpires mistake. What if such a thing happens in wc final last ball.