A unit's charge range is equal to its Movement value plus 2D6 (where you pick the highest score rolled). If the charge range is equal to or greater than the distance to the enemy unit, the charge is successful. If the charge range is less than the distance to the enemy, the charge is failed.
The charge range of a unit is based on its Move value – a faster unit can charge further than a slow unit, as is entirely sensible. That said, it is only right that units can charge further than their Move value, as we're assuming them to be going all-out in order to get to grips with the foe. To represent this, as well as the caprices of fate, a unit's charge range is the sum of its Move value and a 2D6 roll (where you pick the highest score rolled) in inches. For example, a unit with Movement 4 rolls a 2 and 5 for its charge distance, meaning it can charge up to 9" this turn. This can make charges somewhat uncertain at greater distances – just as they would be in real life.
With the unit's charge range established, measure the distance between the charging unit and its target unit at their closest points in an uninterrupted line between them. If the distance is greater than the charge range, then the charging unit realises that if has insufficient momentum to reach the enemy and instead makes what we call a failed charge. If the distance is less than or equal to the charge range, then the charging unit has reached the enemy and makes a successful charge.
If you're charging several enemy units, remember that the rolled charge distance must be sufficient to reach both units, otherwise the charge can only be resolved against those units within range.