About 25 feet is the story I heard also. Plus it was a chance shot.
We, however, recognize the 400-yard figure as a "clerical error", the term used by J. Edgar Hoover to explain Life magazine and the Warren Commission printing Zapruder frames 312 and 313 in reverse order in order to maintain the plausible denial.
No doubt the scene of the Huff shot was chaotic, and the odds against the man trained as a sharpshooter redeployed with a revolver making that shot were great.
But not great enough to prevent it.