How do you know the amount of space being used?
Here is a query you might want to try.

Code:
col name format a40

select
   name,
   to_char(space_limit, '999,999,999,999') as space_limit,
   to_char(space_limit - space_used + space_reclaimable,
   '999,999,999,999') as space_available,
   round((space_used - space_reclaimable)/space_limit * 100, 1) as pct_full
from
   v$recovery_file_dest;