Minecraft wiki entry for Feather_Falling, associated text: With boots enchanted with Feather Falling IV, and at least two other armor pieces enchanted with Protection IV, a player at full health can survive a maximum fall distance of 103 blocks.
The fall distance can be extended to 503.3 blocks provided that the player lands on a hay bale.
Although Feather Falling should negate 12% of the damage per level, at level IV the height of guaranteed death is 55 blocks, not 57-58 (57.5) blocks.