Sneaking or walking off a 64 block high tower with feather falling leaves you with 4.5 hearts left. Sprinting off the same tower kills you. (The game might calculate the horizontal distance also? If it does it shouldn't.) There is no height change between tests. I'm not sure if sprinting totally cancels feather falling out or just lessens it. It should not effect it. Also sometimes shifting and dropping off the same tower kills you.