As you can see from this pic the Harpies are positioned to block the dark riders charging past them. The Dark riders charge the harpies who choose to flee, rolling 12"

The nearest model from the harpy unit moves directly away along a line from the center of the Dark Rider unit. The other 4 harpies move along a parellel path.

The Dark Riders must wheel to go after the harpies, that uses up 7" of their move leaving them only 11". The Harpies are just too far away and the charge is failed

In this example the Spearelves are charging the flank of a unit of Repeater crossbow elves

They are starting their charge from only 4" away.

The RXB Elves choose to flee and roll 5 for their distance. They are pivoted around their center to move directly away from the center of the spearelves. There is an example of this on page 22 of the rulebook.

They are then moved their flee distance directly away.

The Spearelves find themselves out of range and the charge is failed

In our last example the Dark Rider are blocking the Spearelves. The Spears declare a charge and the Dark Riders choose to flee. They roll 7"

The Dark riders are turned to face directly away from the Spearelves and move their flee distance along a line drawn between the center of both units.

The Spears must first wheel after their target and there remaining charge distance if found to be short. The charge is failed.