How accurate was the xat percentile predictor last year
?

Whatever rank you getting out of 34,000 people, multiply it by 2.5 to get your final rank out of 1 lakh aspirants, and then calculate your percentile. For example, my rank is around 2900 out of 34,000 people. So My expected rank out of 100,000 will be around 2.5x2900=7250. Now 7250 out of 100,000 will be around 92.75%ile.

Again this is an estimation based on last year's trend. +/-2%ile variation might happen.