I wonder if this specific measurement was added in several weeks, months after the fact to support the SBT.
That's a safe bet. We know that as of January 27, 1964, "the autopsy" still placed the back wound below the top of the shoulder blade. We know this from the declassified transcript of the Warren Commission's 1/27/64 executive session where Rankin refers to an autopsy photo showing the back wound "below the shoulder blade."
We also know that as of that executive session, "the autopsy" still claimed that a head-shot fragment exited the throat wound. Rankin said that "We have an explanation there in the autopsy that probably a fragment came out the front of the neck." We know from Lt. Lipsey's HSCA testimony that late in the autopsy, the doctors concluded that the throat wound was caused by a head-shot fragment. That's because they had already established beyond any doubt that the back wound had no exit point.