well i am from fiitjee too and this is what my teachers said.
1 - 500 : perctile of around 98 and above will ensure you a rank above 500.
500 -1500 : percentile above 95
1500 - 2500 : percentile of above 90
If your percentiles drop below 90, better double up your efforts. Try to always stay above 95.
And please note that you need to be CONSISTENT. Getting a percentile of 98 once is not enough. Once again, these are just general opinions formed from past year's statistics..so dont bother about this much, just keep working hard...