The ball has to . So if I understand your question, we're really only talking about being in line with leg stump here, as the law states that it needs to pitch in line between wicket and wicket, or on the off side of the striker's wicket. So, for the ball to "pitch" in line, at least half of it needs to be "in line" (between leg stump and leg stump).
However, if you've ever played lower grade or social, self umpired cricket, you know the whole LBW law is a pile of ****. Everything is either "going down leg", "pitched outside leg", or "too high", or the batsman was "too far forward"!