Hi Brian,
You should just be able to draw a sketch line that starts at the face of the B-table and runs .8xx" (B-table offset for your machine) in the Y positive direction. As long as this line is also at the center of B rotation, the top point of it should be an accurate representation of the center of rotation for the machine, which is where you will want to set your WCS.
Hopefully that make sense. Let us know if it doesn't, we are happy to help you further.
Q Rothing
Pocket NC