Well most of the schools give the additional 6th subject for boasting the marks, so for calculation just sum up the top 5 subjects and calculate the percentage.
Keep in mind the change of marks should be between the additional subjects like you took maths then
English maths physics chemistry are the 4 compulsory subjects , if you are bio then change maths with bio and so on.
There are a list of compulsory subjects and elective subjects lists on cbse you can search for that too.
All the best.
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