If above isn't clear (wasn't for me), read article on BBC (clear for me). Or to rephrase it:
Put arm on desk, palm facing up is convenient -- don't move this one. With hand from free (other) arm, point and touch the middle finger of the fixed arm, then the elbow. I did this many times (finger, elbow, finger elbow). Get an idea of how long this arm is.
Now use the fixed arm, pick up a pen, and trace along the screen. I did this for about 10 seconds. Put arm down back into formerly fixed position. Redo the finger-elbow touch with the free arm's hand.
If above isn't clear (wasn't for me), read article on BBC (clear for me). Or to rephrase it:
Put arm on desk, palm facing up is convenient -- don't move this one. With hand from free (other) arm, point and touch the middle finger of the fixed arm, then the elbow. I did this many times (finger, elbow, finger elbow). Get an idea of how long this arm is.
Now use the fixed arm, pick up a pen, and trace along the screen. I did this for about 10 seconds. Put arm down back into formerly fixed position. Redo the finger-elbow touch with the free arm's hand.