You must understand that IIT-JEE or any other competitive examinations do not have any formula that guarantee a certain range of rank. The story is all about how well you assimiliate the informations on your subjects round the years and how skilled are you in conceptions. But above all, it all comes down to the grand finale, the test day. Everything depends on how that day goes for you , the institution where your seat has been allotted, the temperature, your mood, the easiness with your clothes etc. i.e.,  the very minute things which don't matter a lot in daily life are the key components of your success. Maybe if your bite your nails off, it cools you down, so not being able to do so may bring a disaster in spite of good skills and after all, remember, you can say that you gave it your best shot when you feel that you have left nothing that you could have possibly done to make your result better i.e. you feel satisfied with your performance and for that you will definitely need to be in unison with the subjects and be able to do everything you want to on the test 'month'. Don't 'plan' everythig and calculate that, if I do this much right, then I'll get that rank. because nobody knows what it will actually be. So don't speculate, prepare you best and give it your best shot and even if disaster comes your way, take it easy , because life is much more than this.

 

Best of luck.