On nirma website it is wrrtin that cutt off marks is 72% (sum of 50% of board pecentage and 50% of nata marks)

The score which you have seen on the Nirma website is the All India Cutoff. 50% of the seats are awarded through ACPC. If you are going through the All India Quota, you will have to add up your NATA score and board marks and then calculate accordingly. According to the GUJCET, the merit rank will be on the basis of normalised score.