There is a discussion of using the sight-size method on page 84 of Roberta Carter Clark's North Light book,
"How to Paint Living Portraits". Clark calls it "the 'Looking Spot'" but I think she's describing the sight-size method as referred to in this post. It's not a long discussion, but at least the book is still in print (I think!)
As far as copying two-dimensional work is concerned, I often get better accuracy and speed if I turn the source material upside down. I wonder, though, if this is a good "learning experience" when it comes to learning from the masters. Any thoughts on this?
Hybrid Mode
