The more I work on this, the more I am convinced that I need to do some silk shading practice after I submit it on a different project. This technique is deceptively tricky and I feel like it must become fun after you cross the hump of getting used to blending the thread colors and stitch directions. I have not crossed it but I'm getting to that point in learning a technique that I really want to understand it more.
The other thing that I have realized (but I want to talk to Tracy about when I'm in Williamsburg) is that is seems that in order to actually get the shading and color transitions correctly on the stitching, you need to slightly exaggerate it from the photo. It also may just be because I'm using a photo printed from my printer instead of professionally printed.