It does require a 1MB overhead! e.g.

root@ / > dsmfmt -m -data /tsm/stgpool/backup1 200

AIX Server DSMFMT Extent/Volume Formatting Program Licensed Materials -
Property of IBM 5697-TSM (C) Copyright IBM Corporation 1990, 1997.  All
rights reserved. U.S. Government Users Restricted Rights - Use, duplication
or disclosure restricted by GSA ADP Schedule Contract with IBM Corporation.

Allocated space for /tsm/stgpool/backup1: 209715200 bytes

Calculation:

200MB X 1024bytes               = 204,800 bytes
204,800 bytes X 1024bytes       =2.0971520 512-Blocks

Thanks,

Demetrius