Check Flashback Space Usage

A simple sql to show the percentage

SELECT (CASE WHEN PERCENT_USED> 100 THEN 0 ELSE (100-PERCENT_USED) END) PERCENT_FREE
  FROM (SELECT (SUM(PERCENT_SPACE_USED)-SUM(PERCENT_SPACE_RECLAIMABLE)) PERCENT_USED
  FROM V$FLASH_RECOVERY_AREA_USAGE);