i think you wanna ask what percentage in 12th is required and how much marks you should score in JEE mains to be in safe position for adm in so called good college.

if that than you should score 75% or more marks in 12th to be eligible for admission in any prestigious college but its right now in position of changing and than you will not be judge on the basis of your 12th marks for Adm process and it won't be there in eligibility criteria .

for JEE mains i would say it depends upon your state as there is home state preferance and also there is category wise difference so saying anything won't fit better but for general and if you really want to get good branch of your choice than for better iit you jee advance rank should be within 200 rank.

for adm in better nit with better branch for you,you should have scored rank within 20000 that is above 98percentile.

thanks