archiving will take place from many tables, not just 1 header table or top tables from Db02 alone.
there is no one method or excel sheet which can provide answers for space savings.
this is one of the challenges we often face in archiving projects.
another option would be to leverage some of the dashboards from Solution manager DVM Workcenter .
however this will need the managed systems (ex: ECC) to be connected to SOLMAN and also configure various extractors and publish reports.
even with these reports the results are not accurate, as we have tried and gone back and forth with SAP on this.
however with each SP level upgrade looks like they are fixing these bugs.