Bob,
Just FYI.  The internal annular ring requirement may be less but you are
measuring the drilled hole edge to the remaining pad.  You are encountering
material movement/shrinkage and layer to layer misregistration.  With outer
layer annular ring you are registering the photo tool to a drilled hole,
which is easier and more accurate and you get to include the plating in your
annular ring measurement.  Therefore, you do not require larger pads on your
outer layers but every bit helps!
Best Regards,
Scott