TO calculate the overall score both the 12th standard marks and the NATA exam marks are considered.50% of the 12th standard marks and 50% of the NATA marks are considered.The final merit is based on both.So you should get a percent of 85 or above in 12th standard and 140+ marks in NATA exam.

For example your NATA marks are 160 and your 12th percentage is around 87 then

NATA-160/2=80(half because only 50% marks will be considered).

80+87=167

then 167/2=83.5

Your final merit will be 83.5.

If you get 83.5 final merit or above 80 you can easily get seat in ANU.