Hi,
As an answer to your question I can provide you some details how
data archiving statistics are calculated starting from release 4.70
(Please find your answer from below description):
How Is the Database Space Information Calculated?
In write programs, the Archive Development Kit (ADK) automatically
collects statistics. In delete, read, and reload programs, you must use
ARCHIVE_GIVE_STATISTICS to pass statistical information to ADK. The
statistics collected by ARCHIVE_GIVE_STATISTICS can be displayed as
described in Statistics.
ADK calculates the database space according to the relevant ABAP Dictionary object type and uses the function module DDIF_TABL_GET to get the width of the relevant object.
You should note, however, that this width is not necessarily identical with the database space occupied by the object. If, for example, you are using a Unicode database with UTF-16 coding, then, n characters (width =n) in a character-type field occupy at least 2n bytes in the database.
After calculating the database space, ADK assigns each result to one of the following database space categories. These categories correspond to the categories in the Statistics function in Archive Administration. The DB Space is the sum of these four categories.
Please note that this information is an extract from the online documenation (section How Is the Database Space Information Calculated?
in the documentation for the 'Archive Development Kit' ->'Developing Archiving Solutions').
Regarding special treatment of cluster tables you can note 565343 (Statistics: Cluster, pool ....).
The DB space values calcuated for cluster tables is diveded by 5 in order to take into account the compression of cluster tables on the data base. It is better to estimate using the number of lines in the logical tables contained in the cluster (for example BSEG instead of RFBLG).
Hopefully this answer is of some help.
Regards
Ruby.