Hi Belin,
I am perhaps bit of an oddball in the way I do things but I set-up the CAD/CAM engraving work with the X and Y zero's (datum) at the centre of the work.
It is easy enough to locate the centre of the first blank by pencil drawing corner to corner diagonals, the blank is then located on a vacuum table in a corner jig and the centre located by a small laser cross-hair. The Mach3 DRO's are then zeroed and engraving then begins, replacing the finished work with a new blank in the jig, the batch is then completed.
There are many threads here on the forum which discuss various methods and a search should reveal the details but, if it's any help, there is a description of what I have done here
http://www.cooperman.talktalk.net/files/17.htmTweakie.