When we counter bore a PCB the machine uses the back side as a refernce for
which to machine down from, so there for we are limited to the accurecy due
to the varing thickness of the material we buy and use, typicaly FR4
material comes +- .003 to +- .005 for thickers boards, the copper plating is
+- .001 and the sloder mask and final finnish can also have a tolereance, It
is hard to hold any better then +_ .007 in depth and more typicaly  +- .010
but it is even harder to get 2 people to aggre as to the measurement, a
angled counter bore is hard to determen the actual lip and bottom and a
stright counter bore is hard to deternem the bottom as there not real flat.
I have even had to use cross sections to prove how deep a counter bore was.
hope that helps
robert Tarzwell from Megadawn.com, pcb books

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