The top 20 percentile in hsc board as per the trends are

• General category -around 335-355
• OBC-around 350 (upper limit)
• SC-325-330
• ST-320

Now as the percentile is calculated in 500;

To calculate your marks out of 500 you need to consider your marks in the following subjects.
1. Physics
2. Chemistry 
3. Mathematics 
4. Any language 
5. Any other subject
If you have taken more than 1 language, language with the higher marks wil be considered and if you have  more than 1 option for the 5th subject, subject with the higher marks will be considered.
So you can calculate your marks accordingly.

You can calculate and verify from here.

THANK YOU.