set autoextend threshold so that tablespace autoextends automatically after predefined limit

 created a bigfile tablespace using
CREATE BIGFILE DWSG_TBS
LOGGING DATAFILE '+DWSDG_DB1' SIZE 66680M
AUTOEXTEND ON NEXT 41200M
MAXSIZE UNLIMITED EXTENT MANAGEMENT LOCAL SEGMENT SPACE MANAGEMENT  AUTO;

I have some UNLIMITED tablespace and some tablespace with MAXSIZE limit.
I have a monitoring script to monitor the size of tablespaces and ASM diskgroups.
For tablespace, the used=sum(bytes) in dba_data_files and free=sum(bytes) in dba_free_spaces.
This script generates alarm if the used is more than 75% of (used+free),
but at about 85% or so, oracle auto extends. Is there a fixed threshold when oracle decides to auto extend?