In my opinion Tax Audit is applicable if the net total income exceeds Basic exemption limit and the net profit is below 6% /8% even if the turnover is below 1 crore
Tax audit is applicable above 1cr or say 1 cr is the basis or say mandatory rule but secondary rule is if a person profit is above 50 lakh or little bit lower then if he or she wants to do audit or wants to maintain books of account they can did fully depends upon the client