Hi there. 

Percentile is calculated depending on the total number of candidates who have written the examination. 

For example, if I say that "Ram got 68 percentile in his examinations" it means that "Ram has scored better than 68% of total candidates who wrote the examination." 

Hence, scoring 500 marks may or may not make you stand at 50th percentile because that isn't how the principal of percentile works. 

Hope you find this information helpful. 

Feel free to post further questions here. 

Good luck!