This was discussed at length
in this thread.
http://www.yorkietalk.com/forums/gen...ocking-16.html
I linked to the part where your question is best answered.
Personally, I have my vet dock at about 1/4" into the black. I get consistently good results giving him that instruction. When I say 'into the black', I mean that the cut is made about 1/4" into the black as measured from the end of the brown on the underside of the tail.