Been there. Asked that.
CBSE only takes 5 subjects into account for its pass criteria. It depends on whether you are calculating your percentage to apply to a college or just telling your relatives and friends about it.
If you are calculating your percentage to tell your friends or relatives about it then you can add up your top 5 scores and divide it by 500 and multiply by 100. This is your best of 5 score.
But if you are applying for colleges then you have to see what criteria they require. If the ask for percentage of all subjects then you have to take the average i.e divide by 600 marks but if the said college requires specific subjects then you can cal