I'm not sure whether JEE board is considering 12th % + JEE performance in JEE ranking
Till 2009 , thr isnt anything like that .. if thats is wat in this year too continues , it shld not be a problem
To be eligible for admission through IIT JEE you should have passed class 12th or equivalent with atleast 60% marks in aggregate. There is 5% relaxation for SC/STs too.
I think you can correct that at your counselling center once uyou are through IIT JEE.
You can check your appplication status @ http://jee.iitd.ac.in/app.htm
If you feel that u r not comfortable, contact an IIT oofice of your zone through phone or fax your 12th corrected percentage by sending a duplicate 12th grade card ...
you can fine contact details in JEE section of each respectuve IITs