Estimating initial tablespace size

I need to create a new tablespace in one of the databases and I wanted to see if there are best practices for estimating an initial tablespace size.

I looked at an existing tablespace that is serving similar purpose and the size of such tablespace is about 25000M (size of all its datafiles) and block-size is 32768.

SELECT TABLESPACE_NAME, SUM(BYTES)/1024/1024 AS TBS_SIZE
       FROM DBA_DATA_FILES
       WHERE TABLESPACE_NAME ='TABLESPACE1'
GROUP BY TABLESPACE_NAME;

based on this information how cand I figure out the best initial tablespace size?