The recommended value for the PARALLEL_MIN_PERCENT parameter is 0 (zero).

This parameter enables users to wait for an acceptable DOP, depending on the application in use. Setting this parameter to values other than 0 (zero) causes Oracle to return an error when the requested DOP cannot be satisfied by the system at a given time. For example, if you set PARALLEL_MIN_PERCENT to 50, which translates to 50 percent, and the DOP is reduced by 50 percent or greater because of the adaptive algorithm or because of a resource limitation, then Oracle returns ORA-12827. For example:
SELECT /*+ PARALLEL(e, 8, 1) */ d.department_id, SUM(SALARY)
FROM employees e, departments d WHERE e.department_id = d.department_id
GROUP BY d.department_id ORDER BY d.department_id;


Oracle responds with this message:
ORA-12827: insufficient parallel query slaves available