After reading in the OTN thread Does tha UNDO Ts count on Database Storage Limit ? I tested my own instance using Paul M.s SQL statement:

SELECT total
     , allocated
     , allocated / total * 100 percent_free  
  FROM (SELECT 5120 total
             , SUM(bytes) / 1024 / 1024 allocated  
          FROM dba_data_files  
         WHERE tablespace_name != 'UNDO'
       )
;

     TOTAL  ALLOCATED PERCENT_FREE
---------- ---------- ------------
      5120       1510   29,4921875

SELECT total
     , allocated
     , allocated / total * 100 percent_used
  FROM (SELECT 5120 total
             , SUM(allocated) / 1024 / 1024 allocated
          FROM dba_data_files df
             , (SELECT tablespace_name
                     , SUM(bytes) allocated
                  FROM dba_segments
                 GROUP BY tablespace_name
               ) sg
         WHERE df.tablespace_name  = sg.tablespace_name
           AND df.tablespace_name <> 'UNDO'
       )
;
     TOTAL  ALLOCATED PERCENT_USED
---------- ---------- ------------
      5120  1230,6875   24,0368652

Just a minor change on the [bookmark] section of my blog, instead of IDs I'll provide unique names, with an optional version tag later on. So instead of ::::P3_ID:<ID> the link changed to P3_NAME:<NAME>. This required of course some (minor) changes on the APEX page and on the pipelined function.