birly-shirly
New
My assumption is from his own experience, but as birly mentioned, maybe he got it from Jack or someone else.
Birly, I've got 2 Nicklaus books, unfortunately Golf My Way is not one of them.
Dan - Golf My Way is highly recommended. You can take or leave his swing thoughts - his full swing technique is a relatively small part of the book. For my money (although I think I bought it second-hand!) it's maybe the best player's book in terms of how he approached practicing, playing and competing. Of course, he probably gets the ball-flight rules wrong - but there's a wealth of information there, some of it quite unconventional.
The putting thing was a description he gives of playing around with stroke path and the degree to which he opens and closes the putter face until he's happy with the way he's rolling the ball and his feel for distance. He was clear that he would adapt different strokes to different greens conditions. He comes across as more of a feel player and shotmaker than maybe the popular conception of him would have you believe.