Remember that it’s not the fall that kills, it’s the sudden stop. So tracking time in air won’t be enough, as people said previously you have to track velocity changes. I personally suggest to track velocity magnitude change (i.e. all directions). This way damage will apply even if you get slammed against a wall.
One thing that may or may not be useful - be careful when doing your calculations on ragdolls. Their velocity is pretty ****ed up and can give wrong readings.