It will depend on the focal ratio as to how precise it needs to be for imaging but here is a link dealing with the just what your saying.
It's a large file and only the first page is relevant but he is saying that for his f4 astrograph his deep of focus is .0015"
It will be less at f5 .
That works out if i am correct at 0.038mm??
http://www.clementfocuser.com/images...Clementrev.pdf