This falls into a zone where madness lies. Option 1: 5d6 damage (full fall). Option 2: 2d6 damage (20 foot fall). Option 3: 0 damage (no fall). If you try to figure out which is right, you have to start applying "physics" to magic, and that is inherently going to result in contradictions, disagreements and frustration. Accordingly, I udsually avoid this by turning to the dice.
Tim: "I just off the cliff, and once I fall 20 feet I Misty Step to the ground. Will I take damage from that? If so, how much?"
DM: "You're normally going to take 2d6 from the 20 feet of falling, but you could try to adjust the Misty Step spell as you cast it to arrest the momentum. If you get it right, you'll take no damage. If it is a little wrong you'll still take 2d6. If you botch it, you may end up taking 5d6. It is a pretty hard trick to pull off. It would be an Arcana roll."
Tim: "I'll go for it. Ouch. Natural one for a 7."
DM: "You're timing is off and you smash into the ground full force, taking the full 5d6 for ...14 damage."
... or ...
Tim: "I'll go for it. Nat 20 for a 26!"
DM: "You time it perfectly and manage to arrest your momentum as part of the spell. You hit the ground on your feet. You can do a Black WIdow or Iron Man Super hero landing if you'd like."
Tim: "Widow. I'm a poser, eh?"