At
http://www.rabble.ca/babble/ultimate...&f=14&t=000404, a discussion forum at Babble, scroll down to Jim Rodger's post.
He sez: "'Close enough for government work.' (First heard from artillerymen of the 4th Infantry Division, An Khe, Vietnam, 1970, upon shelling the wrong village or trail or whatever.)"
I have not been able to verify this elswhere.