But this is probably an overestimate.
Just to show how the perspective can introduce additional angle into the throat-back wound trajectory, I generated in image of the operating table in a ray trace program (POV-Ray).
These images are not meant to represent the actual measurements that exist in the real autopsy table environment, for I don't have those, nor do I have information about the camera lens, its distance to the table, its orientation, etc. However, I have tried to position the virtual camera so that the angle of the upper edge of the table and the dark stripe on the table matched the photo. The viewpoint has been rotated to match the vertical orientation of the Artwohl diagram.
The left image is from a viewpoint that about matches the viewpoint of the camera in the left profile autopsy photo. The blue stripe corresponds to the dark stripe on the table. The brown stripe represents the approximate level of the trach incision on the table. The green stick sticks straight up out of the table at this point. It represents a zero degree angle from the trach incision to the back wound. Notice that because of our perspective, the stick appears at an angle--I measure around 13 degrees.
For the right image, I moved the camera down to the level of the brown stripe just to verify that the green stick really is perpendicular to the surface of the table.
In another test (not shown), I tilted the green stick downward until I matched Artwohl's 21 degree angle. I had to tilt the stick 7.4 degrees to get the 21 degree angle from the autopsy photo camera perspective. Note that this 7.4 degree is not necessarily the "true" angle since my model might not match the autopsy setup exactly.
One other problem with the Artwohl diagram is the 5.5 cm line he draws from the neck crease to locate the back wound. This line too needs to be corrected for perspective. As it stands, Artwohl's intent was to draw it so it extends inferiorally, parallel to the spine, but I believe because of the perspective, it is drawn more laterally across the shoulder. This will mislocate the backwound, giving the appearance of an increased downward angle.
None of this is meant to analytically identify exactly what is wrong with the Artwohl diagram, but only to point out factors that Dr. Artwohl may have overlooked.