Hi Viveka,

Top 20 percentile is calculated by taking into account FIVE subjects-

• Physics
• Chemistry
• Mathematics
• A language
• Any other subject apart from the four mentioned above

You need to calculate your percentage out of 500 i.e. the marks obtained in the five subjects mentioned above.

I can provide you the Top 20 percentile cutoff for HSC Board for OBC-NCL candidates from the previous two years, you can get an idea.

In 2017, 

Cutoff marks- 344 out of 500 (68.8%)

In 2018,

Cutoff marks- 362 out of 500 (72.4%)

The Top 20 percentile list for all the boards will be released by JoSSA later this month or in early July.

Hope this helps you.

All the best!!!