Alphonse Mucha used the time-honored method of scaling up with a grid when he wanted to translate his reference photo to his finished cover illustration.
He drew a series of evenly-spaced horizontal and vertical lines directly over his black and white photo. He then added another set of diagonal lines to subdivide the grid in crucial areas of the face.
Presumably he redrew this grid on a separate piece of paper and then copied the content of each grid square to develop his comprehensive drawing. This separate drawing then would have been his planning step that he would have transferred down to the final painting (shown here as a black and white photo of the magazine cover).
Whether you're working from a reference photo or a hand-drawn figure study, scaling up is still one of the fastest methods, and it has a lot of advantages over eyeballing or projecting an image.
It assures that you've got the proportions and placement exactly right, and at the same time, it lets you feel more in control of the drawing, changing and improving on the reference. Mucha, a master draftsman, certainly used the photo here only as a starting point, and like Rockwell, took it in his own creative direction.