Hugh Sprunt took an interest in this and borrowed a human skull which he analyzed to determine if a point 7 1/2 inches from the top of Foster's skull would place the entrance wound on Foster's posterior oropharynx (it did).

Sprunt's methodology was to use a human skull and scale it to Foster's skull (Sprunt's findings and methodology are documented on Allan Favish's web site, on  the ANATOMY OF THE HEAD WOUND page)

I decided to check Sprunt's location of the entrance wound independently of measuring skulls and scaling one to the other.  Here's the process and the result.

According to the FBI Lab report dated May 9, 1994 [245 in the Senate Hearings] two sets of Foster's eyeglasses were inspected by the FBI Lab. 

The Lab reported that PD (Pupillary Distance) for these glasses was 71 millimeters and 73 millimeters. 

Since pupillary distance (PD) measures the distance between the pupils of the wearer, we know that the distance between Foster's pupils was between 71 and 73 millimeters (i.e. between 2.79 inches and 2.87 inches)

(click on image for larger view)

I scanned in a fairly frontal view of Foster from Ruddy's book (The Strange Death Of Vincent, p 157), drew a line between Foster's pupils and measured it. The horizontal red line measures 101 pixels (on the larger version of the image). 

If we assume that this line represents the 71 mm PD then the line represents 2.79 inches (71 mm * 0.03937 inches/mm). 

101 pixels per 2.79 inches   = 36.2 pixels per inch

(click on image for larger view)

I then guestimated the crown of the head by merely drawing in what looked to be a fairly natural curve, ran a horizontal line across this ersatz crown and measured down 271 pixels (7.5 in * 36.2 px/in = 271.5 pixels) to get an approximation of where the entry wound would be if projected onto Foster's face.

The figure on the left shows the result.

(click on image for larger view)

If we use the 73 mm PD as the distance of the horizontal line between the pupils, then we have 101 pixels per 2.87 inches = 35.1 pixels per inch, and can run another line (blue) from the ersatz crown line, representing 7.5 inches at 35.1 pixels per inch or 263 pixels in length.  This result is shown on the left.
 

We can then estimate the location of these points on the lower lip of the anatomical drawing, and draw a horizontal line towards the back of the head to see what parts of the head lie on a point 7 1/2 inches below the top of Foster's head. The result is shown below. 

The blue line represents the 7 1/2 inch distance as scaled to the 73 mm PD. 

The red line represents the 7 1/2 inch distance as scaled to the 71 mm PD. 

The green line represents the 7 1/2 inch distance as scaled to the average of 71 and 73 or a 72 mm PD.

As you can see a point 7 1/2 inches below Foster's head is right on the posterior oropharynx, exactly where Dr. Beyer reported it. 

Sprunt rendered his findings on the same anatomic diagram, which may be viewed on this page of Favish's site.  For convenience of comparison, I inserted a cropped version of Sprunt's drawing below, followed by a copy of my image.. 

Sprunt drew two lines.  The red line in Sprunt's drawing is his "best guess" estimate of where the 7.5 inch location of the entrance wound as described by  Dr. Beyer (see ANATOMY OF THE HEAD WOUND for a description of the "best guess" line and how it was obtained)

Although Sprunt and I used completely different methods, our results are in excellent agreement -- i.e. Sprunt's 'best guess line' is within one pixel of the line representing the average of the 71 and 73 mm PD measurements. 

The image below illustrates:
 

• I took the images above and put in a Green line representing the location of the entrance wound, assuming an average pupillary distance of 72 mm on the Huntington image. 

•  I then copied fragments of both images into a new image. 

• Next I ran a horizontal line between the upper and lower lip of the Sprunt image and aligned the Huntington image so this line (labeled 'Alignment Line') passed through the lips of the Huntington image, thereby aligning the two images. 

• Finally I cut out a portion of the resulting image, enlarged it and pasted back.  This enlarged view will enable readers to see the alignment between the 72 mm line on the Huntington image, with Sprunt's Best Guess Line on the Sprunt image.