There is no minimum required percentage to sit for IAS Entrance exam, only graduation is enough to sit in the examination.

In the order to become an IAS officer you need to appear in the Civil Service Examination, this examination is conducted by the UPSC. Here I am giving you the details of this examination:

Nationality:
A citizen of India, or
A subject of Nepal, or
A subject of Bhutan, or
A Tibetan refugee who came over to India before 1st January, 1962 with the intention of permanently settling in India, or
A person of Indian origin who has migrated from Pakistan, Burma, Sri Lanka, East African countries of Kenya, Uganda, the United Republic of Tanzania, Zambia, Malawi, Zaire, Ethiopia and Vietnam with the intention of permanently settling in India.

Required qualification:
The applicant should hold a degree of any of Universities incorporated by an Act of the Central or State Legislature in India