X0 should be the center of the part regardless of diam. or rad. mode.
In diam. mode when the tool is at center and you do "G0X25" the tool should move 12.5 increments because you are referencing the diameter of the part.
In rad. mode you must do your own math after measuring the diameter of the part.
I can't even guess why a tool would go to the rear radius for X zero.
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