so did you do this from a photo or was Mr. Whitman stuck on one of those flight-delay-layovers-from-hell in the airport?
... from a photo and used the iPad's split screen. The reference photo was on my left and my drawing on my right.
iirc, you've watched the movie "Tim's Vermeer"? remember his initial experiment using the B&W photo (of his father-in-law?) and a mirror, then just matching the paint to the picture? i wonder if you could something like with an iPad?