Performing actions while falling

If a character falls from a great height and has a skill or item that may save them if used early enough, how does one determine where in the fall the item is used? Depending on the reason for the fall, there may of course be some DC to avoid falling in the first place (usually a Reflex check against the DC of the trap that’s opening up beneath you) but what I’d like to know is what happens once it’s decided that the character is definitely going to fall.

For example, say I have some kind of teleportation effect that can get me back on solid ground, but the rules say that it doesn’t affect my momentum, so if I’m falling at speed I’ll still hit the ground hard when I emerge wherever I teleport to. So, this will only save me if I can do it fast enough – for D&D3.5e that’s basically before I’ve fallen ten feet (or twenty if I can make a DC 15 Jump or Tumble check to negate damage from the first ten). How do we determine if I manage that? Can we assume I make it if the activation of the effect is fast enough? Is there another check? My instinct is that it’d be another Reflex check, but I’m not sure what the DC would be.