Percentile is a measure of how much you score with respect to the highest scorer irrespective of how much marks you scored. So, you can have a percentile of 50 it can be possible that you actually scored 0 marks out of 300. Also, people can have a positive percentile even if they have a negative score. 

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Percentile is a relative thing.

For eg . Who gets highest mark in cat 2017 will get 100 percentile( normally 5 -6) every year.

Those who gets 10–15 less than toppers falls in 99.99 category

All candidates with difference of 0–50 with respect to toppers falls in category of 99.99–99.01 ..(approx) and so .

Percentile depends upon your rank.

In short

170–99 percentile

150- 95 marks

120–90 percentile and soon

Approx calculation based on last year trend.

Percentile of a student refers to the percentage of students below him in the overall test.

The point of having percentile is: Irrespective of the difficulty level of the exam you have the chance to come up with the final percentile as high as maximum.

Paper will remain difficult for all, paper will remain easy for all and therefore the score is dependant solely on your preparation and performance.

I hope this gives you a sensible solution.