Hello sadique,

75% in 12 board is a precondition for neither jee mains nor jee advanced.75 % or top 20 percentile  in respective board is needed for securing a seat in government funded institutes.It is only considered during the time of admission.

Though there is no precondition of marks to sit in jee main,there is no point in attempting jee advanced if you have scored less than 75 % and are not even in top 20 percentile of your board as you will not be getting admission in IIT's .

Hope it helps.