How does one make Deashi Harai a higher percentage technique?

I was wondering if anyone had any good ideas on how to make the move Deashi Harai a higher percentage technique? The timing must be exact, so I was wondering if there were any ways to get closer to the correct timing apart from a random shot in the dark? Such as movement to watch out for, or moving your opponent in such a way as to get you at least closer to the mark.



By the way, Deashi Harai is this move:

http://ift.tt/1kwLSe5



I noticed when searching for videos that some did the move instead of sweeping the foot completely perpendicular to the step, they seemed to sweep the ankle forward. I am interested in both techniques, but there is a distinction between the "ankle hook" and the perpendicular "foot slide/sweep."