Reference: Charging, Page 23.

Players are not permitted to measure to their intended target when they declare a charge. A player must use his judgement when declaring a charge, because if the target is too far away he will only be permitted to make a normal move and also loses any opportunity he might otherwise have to shoot with missile weapons. On the whole, a failed charge is bad news for the would be charger; not only might he suffer unnecessary 'stand and shoot' hits but he will be vulnerable to an enemy charge in the next turn.

When he declares a charge a player must be reasonably sure that his unit will reach his target. A player cannot attempt to gain an advantage by declaring a charge if it is immediately obvious his unit cannot reach its target. In most cases this will be disadvantageous, but it is possible that an advantage might be gained in some situations. Where it is blindingly obvious a unit would be unable to reach its target a player is not permitted to declare a charge.