Hey sandy

You can not calculate your marks because percentile is a relative score not exact score.

It is the comparative score of you among all other students in your slot.

It is based on the number of students who score more than you.

Like in a test there are 200 students and you score 150 marks and 100 students score more than you 

Case 2 : you score 250 marks and 100 students score more than you .

Than in both case your percentile remains same but marks are different.

I think you get it.

Any other query ; you can ask freely.

Good luck