I know this rule is rarely see. I. Lizards but I have started my second army, orcs and goblins, which uses it a lot! Where it says if unit is In range it must charge. But what dictates a unit being in range as it is determined by a die roll. Also if a frenzied unit moves in range of a unit does it then charge or await its next turn?
The best posible roll of the dice while charging, is considered in range. (Frenzied units can only charge in the declare chargers phase)
Note that a frenzied unit does not have to charge if it is within range, it is simply required to check to restrain the charge with a LD test. For this reason it is critical to have a BSB since a favourite tactic is to force frenzied troops to declare unlikely charges so that they only move a few inches forward rather than being able to march. Also, keep in mind that frenzied troops must always overrun and pursue so it is generally a good idea to keep this in mind before committing them to a combat where they could end up with an exposed flank.