To qualigy to make a flank charge a BG must have at least one of its bases wholly behimd the line extending the front edge of the targte BG (and no elements in front of that BG) - however, the rules do not say that the base that contacts the enemy flank edge needs to be a base that was entirely behind this line.
Am I reading this correctly?
If so we played it wrong in one game at the weekend - mind you it was a bit confused in that the base that was behind the line actually contacted another BG, also it made no difference to the game.
