hi zaheer , 

it all depends on the difficuilty level of the question paper which will inturn dcide the cutoff range in jee mains . however , to be sure of getting an iit seat it is better to score above 300-320 to be definitely sure about your seat in iit .

all the best 

hello zaheer

there is no fix marks your rank should be high enough to excell you through the counselling session as the difficulty level often changes

hope it helps