First of all for which exam you are doing this calculation that has not been mentione here. And the weightage is for the class 12 including the PCM marks which is a common practice for most of the entrance exams. But it is not our job to do such things because the calculation of weightage is not a predefined process rather it depends upon the type exam you are giving after your class 12. You must have scored atleast 50% marks in class 12 to get into any examination or institutes. There can be a different ratio of the weightage for you class 12 and class 10 so it is not the thing to be worried about. At the end your merit rank is what that matters. Based on your merit rank you will bw getting the admission in the institutes.