To calculate 12th marks, you just have to add marks of all the subjets and divide the total by the number of subjets.
Like let me give an example by calculating my percentage:
1 Eng 86
2 Physical education 97
3 Business studies 95
4 Economics 97
5 Accounts 95
And the total is 470 and divide this by number of subjets and that is 5 and so 470/5 = 94 percentage!!