Classically the arm, and the thigh on the same side, are about 30-45° from your torso.
There are very practical reasons for this.
Such a strike is always short, quick and down. You never have to think about the direction of the fall, as it is always down, downward toward your midsection.
Try it yourself, just lying on your back. Hands roughly above your midsection, drop them down to slap 30-45° from your torso, then 90°, then 135° or so.
As the angle increases, the distance greatly increases, and the arm is rotated so that your elbow is exposed and subject to injury, then your shoulder joint is stressed a lot more.
Different angles, different timing mean that you have to time and think about it.
Same angle every time, no thinking, just ukemi.
If you're stretched out and think that you need to reach above your head, perhaps time to think about tucking and rolling in that direction.
Last edited by Lance Gatling; 1st January 2018 at 00:28.
Lance Gatling ガトリング
Tokyo 東京
Long as we're making up titles, call me 'The Duke of Earl'