Calling back a TD for a block in the back well behind the play should only happen if the contact was significant enough to rise to the level of UNR. Ordinarily, a block in the back is not considered a safety foul, and therefore, should only be called when it places the defense at a disadvantage. My suggestion would be if the contact is sufficient to warrant a flag, call it UNR, not a block in the back. There is plenty of rules support for this.
9-4-3
“ART. 3 . . . No player or nonplayer shall:
b. Charge into or throw an opponent to the ground after he is obviously out of the play,”
I think 30 yds. behind the runner qualifies as “obviously out of the play.”
On the forum where I got this play, you would not believe the number of officials who said the block in the back should be flagged and the TD called back.