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 & 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 obvious a unit would be unable to reach its target a player is not permitted to declare a charge.
Moving chargers is perfectly straightforward in most cases. However, a battle being what it is, fought over infinitely variable terrain and subject to the strangest of circumstances, charging can sometimes degenerate into a tangle of units scattered all over the countryside. If players bear in mind the following guidelines they should be able to sort out even the most complex situations.
Once the charging unit has touched its opponent, the combat is committed and it only remains to align the antagonists where the charge has been made at an angle. Bear in mind what is really happening. The two units have clashed and some warriors in advance of the rest have struck the enemy first, but the rest will pile in beside them and gradually both sides come together into a wave that ebbs and flows as the troops fight. For the purposes of the game we simply move the combatants against each other so that a battleline is formed, and for convenience it is easier to move the chargers. Therefore, the charging unit is aligned to its target, but in situations where it would be more convenient (eg, if impassable terrain is in the way) it is entirely possible to move the charged unit to align it with the charger.
This extra 'wheel in' happens automatically and can indeed result in some models moving considerably further than their normal charge distance – this is absolutely fine by the rules.