N : total number of candidates appeared for exam.
(N- Your rank) indicates the number of candidates who have scored less marks than you.
Total Appeared is 18000
My GATE score is 409
P = ( (18000-100)/18000 x 100 = 99.44 %
so your percentile is 99.44
Note: Still IITK does not released total number of candidates appeared for exam. Wait for a while.
As per our experience approximately XL -10,000 and BT - 20000 students might have attended 2015 GATE Exams.