The rules may differ from university to university. Considering a general perspective this can be the process to calculate the index mark:
In the case of candidates passing their qualifying examination other than Degree Examination of University of Calicut, for the purpose of calculating index and ranking for admission to the P.G programme, the total marks/grades(CGPA) obtained by such candidates at the qualifying examination/ specific subject shall be brought to the corresponding aggregate total / subject total/grade (CGPA) of the Degree Examination of University of Calicut, unless specified otherwise.
5% of the selection index should be added as grace marks/point arrived at for preparing the merit list for admission to Post-Graduate programmes in the case of those candidates who passed their qualifying examinations from Calicut University except .
Students who have worked in the Illiteracy Eradication Programme on voluntary basis in any capacity for at least one year will be allowed 5 bonus marks/points for PG admission
Five (5) marks/points shall be awarded as bonus marks in respect of N.S.S / N.C.C cadets for admission to Post-Graduate programmes,
NCC cadets holding A, B, C certificates are eligible for an additional weightage of 3, 5, 10 marks/points respectively for the purpose of admission to Post Graduate programmes
Bonus marks or additional weightage marks/points will be given only in the case of those NCC Certificates on the basis of which no grace marks/points have been awarded already and the additional weightage will be limited to a maximum of 10 marks only.
A weightage of 10 marks/grace points (to be added to the aggregate index marks/points) may be made to those candidates who have successfully undergone the CSS Programme of this University for admission to PG programmes.
A deduction of 10 marks/points for every successive attempt after first attempt will be made out of the ranking total, if one has taken two or more chances to complete the qualifying examination, subject to a total of 25 marks/points. The number of chances means the number of chances taken for passing any part or parts of the qualifying examination. Betterment of results will not be considered as a chance.
Bonus / weightage marks will be added to the index marks/point for ranking in the selection list in the case of candidates who already possess the required minimum marks and qualifications prescribed for admission.
I hope my answer helps you. All the very best for your future endeavors!