Farthest Reachable Destination

Hello Guys:

I'm new to Unity's navigation system, and I'm building a game that the enemies would try to flee from the player if their HP reached some thresholds.

And currently, the scripts I used to define the flees are :

 Vector3 Source = DamageMessage.damageSource;
 Vector3 Direction = gameObject.transform.position - Source;
 FleeDestination = gameObject.transform.position + Direction.normalized * 15;
 FleeDestination.y = 0;
 m_NavMeshAgent.SetDestination(FleeDestination);

The enemy would run for a fixed distance in the direction that opposite to the damage source, but sometimes this would result in a destination outside the Navmesh area, and the Navmesh agent would just infinitely run to the destination in a corner of the Navmesh area.

So I'm wondering if there is a way that I can set the destination to the farthest reachable location within the Navmesh area in a certain direction and distance? Any ideas?