The quality of the mid-sag plane estimate will, of course, depend on the
quality of the points you or someone else chose when aligning the volume.

At the very, very least, I would look at a lot of your volumes, frex paging
through the axial slices, to see how accurate you feel the plane is. But you're
probably better than I am at the alignment.

Another question I have is how you allow for an even number of slices--
I don't know which one AFNI will choose to display. This would definitely
affect how consistent your area metric is.

If this procedure is not consistent enough for your needs, I saw a
set of papers at the HBP Neuroinformatics Conf that give an
algorithm for a robust, consistent estimate of the midsag plane, even
in the presence of tumors, etc. I can look up the details if you need
it, and it shouldn't be too hard for somebody to implement.

rich