You could run a backup stg command prior to running your normal backup stg
and that would give you the total amount of data that needs to be copied
from one storage pool to another.  The command would be:

ba stg tapepool copypool preview=yes maxpr=1 wait=no (or wait=yes)

This would give you one process that would tell you the total amount of data
that needs to be backed up from one to another and then you could watch you
real ba stg process to see how much data they have accually moved.  The
preview does not require tape mounts so it is preaty fast.  I use it on
occassion to see how much data was written directly to tape (since I have
backups that go to disk and to tape)