I would reduce it by 10' if you weren't prepared for the fall, but by much more (30-40) if you jump or dove.
I just reduce the distance you fell by 10' if you fall into water. So, falling from a 10' ledge into water isn't really going to hurt you. But, from a 100' bridge. Oh yeah, it's still going to hurt.
Naturally, you can still make an Acrobatics check to reduce the falling damage.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.