Was that drawing too big for this forum? I need to get an idea of the shape you are working within. A simple surface model would be adequate ![Smile](images/smilies/smile.png)
But generally, your question would be answered by this: ball mill toolpaths are calculated from the center of the ball radius, not from any point on the circumference. This is contrary to using a flat ended mill, where the Z datum of the cutter is on the bottom plane.
First you get good, then you get fast. Then grouchiness sets in.
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)