[BattlemageBob]

damage is not force, damage is the effect of the application of force.
example: Hand to hand, punching somebody in the face does not cause the same amount of damage to both people.

Damage caused by falling will be determined by the weakest spot where force is exerted in the system. For the person getting attacked from above this will likely be the neck, so I would apply damage there to the person being jumped on.

To determine damage for your situation I would first determine if the attacker was able to hit and how well he hit, most likely using the flying kick skill and modifying by the enemys movement speed and the distance your player is jumping from.

Next determine if the enemy was aware of the attack. In real life a lot of force applied to your neck if unsuspected will easily kill a person. If he is aware he will tense his muscles causing it not to be an auto crit. If unaware, treat it as a critical hit.

If the hit isn't enough to knock out the enemy, then your player takes normal falling damage at that height to the leg. Then roll acrobatics to see what happens when he falls from his target at the height of the target and determine additional damage to the player.

If he was able to knockout or kill the target, then he has a slightly softer landing. I would subtract a percentage of his velocity based on how much damage he did to the enemy due to the enemies bones and muscle giving way to the kick and slowing your down before you hit the 3rd derivative of position. Then figure out damage to him based on the new velocity and an acrobatics fall from the new height.