怎样才能正确计算Oracle数据库表空间的利用率？

SELECT UPPER(F.TABLESPACE_NAME) "表空间名", 
D.TOT_GROOTTE_MB "表空间大小(M)", 
D.TOT_GROOTTE_MB - F.TOTAL_BYTES "已使用空间(M)", 
TO_CHAR(ROUND((D.TOT_GROOTTE_MB - F.TOTAL_BYTES)
/ D.TOT_GROOTTE_MB * 100, 
2), 
'990.99') "使用比", 
F.TOTAL_BYTES "空闲空间(M)", 
F.MAX_BYTES "最大块(M)" 
FROM (SELECT TABLESPACE_NAME, 
ROUND(SUM(BYTES) / (1024 * 1024), 2) TOTAL_BYTES, 
ROUND(MAX(BYTES) / (1024 * 1024), 2) MAX_BYTES 
FROM SYS.DBA_FREE_SPACE 
GROUP BY TABLESPACE_NAME) F, 
(SELECT DD.TABLESPACE_NAME, 
ROUND(SUM(DD.BYTES) / (1024 * 1024), 2) TOT_GROOTTE_MB 
FROM SYS.DBA_DATA_FILES DD 
GROUP BY DD.TABLESPACE_NAME) D 
WHERE D.TABLESPACE_NAME = F.TABLESPACE_NAME 
ORDER BY 4 DESC