for me, it doesn't matter that much on when and where. i have worked with the strike from starting point all the way to almost hitting me and anywhere in between. it's a game i play actually just to see if i don't bang the arms, still disrupt uke's balance, and stay relax at the same time. my favorite spot lately has been waiting for the strike to almost hit my head, then bring my arm up to deflect and not moving my feet at all. sort of a wingchun version then follow with a shiho nage or whatever depends on uke's balance. of course, now folks would scream "That's not aikido". my answer would be "that's my aikido. go away!".
That is more like what I was getting at.