... and I'm also trying to open the hand out at the knuckles, rather than just use the fingers. Chang refers to this as 'stretching the palm'.
I am not so sure what you mean by this. Furthermore, stretching the palm is not stretching at the knuckles! The fingers start at the wrist. Stretching the fingers at the wrist will give the largest span, so opening the hand should always start at the wrist. Stretching at the knuckles is possible, but rather difficult. It is usually forced rather than a controlled motion, e.g. when playing a fourth with fingers 2 and 3, one of the fingers is already is usually already on the key, while the other one "reaches" out for the next note.