Widest Part at Waistline
A properly fitted tie's widest part will hit the belt line of your trousers with the tip overlapping your waistband. Using this method, your shirt should not be visible on the sides of the point.
Tip at Waistline
Some advise that the tip of your tie should touch the top of your belt. Using this method, your shirt will show on either side of the tip.
The correct length for a tie is for the widest part to hang even with your belt line. When tying a tie, it is best be err on the side of a bit too long rather than a bit too short.