In reply to Nebulous:
You always fall the same distance regardless of weather you clip at the wiast or above head height, however you will finish lower if you clip above your head which could have implications if the ground or a ledge is nearby!
Quick example, 2m between bolts, clip the lower one, climb so my waist is level with the upper, I will fall 4m, finishing 2m below the bottom clip...
same bolts, clip the lower one, climb so my waist is one meter above teh bottom clip(exactly in the middle), reach up and go to clip the top one, I have 3m of slack at this point (2 from lower clip to location of upper clip, and then one back to my waist). I fall, I'll drop 1m till i'm level with the lower bolt, and then the further 3m of slack. Total fall 4m, although I will now be 3m below the clip.