I think whoever made this graphic may have made some slight errors.
First of all the 14 mark is about half way and should be 15.
Secondly the mastoid process should be a bit higher than the top of the ruler in this graphic, and furthermore as the HSCA pointed out, the mastoid process is movable when compared to surrounding landmarks but I believe the 14cm measurement would logically be taken when JFK was in this position?
Thirdly the ruler is angled slightly downwards and away from Kennedy's head.
Fourthly the autopsy photo doesn't show crystal clear ruler numbers but based on the blurry 1 cm black square at the bottom, I measured about 27 or 28, so the ruler could be 25 cm or perhaps 30?
So taking all this into account the bullet hole being 14cm down from the mastoid process is a close enough fit! But at the end of the day numbers on a page are relatively important for imagining scale but as compared to an actual photograph where we know the dimensions of the person these numbers aren't as important, because as they say a picture is worth a thousand words!

JohnM