Since Ansel spent most of his time in California, I imagine that is where he came up with it. That is where his darkroom was and I do believe that someone who is faced with developing and printing his own images, that he himself took is more likely to have such a conception. On the whole idea of the Zone System one should remember that he had a colleague in this effort: Fred Archer. My recollection is that Fred and Ansel taught together and their desire and need to communicate with students probably had much to do with the clarifying the conception and practical aspects of Zone System techniques.
As a side note I find the term previsualize to be very curious. It is almost like saying visualizing prior to visualizing. It make me wonder if Yoge Berra helped coin this term.