what's the percentage have to get to get into IIST in Mains??
Mains marks has nothing to do with this. JEE advance marks are considered for the admission in IIST.
Basically Advance question pattern is not fixes so i am considering previous year cutoffs with previous year pattern.
For general category you need a minimum of 35% in JEE advance to get Aerospace or Electronics and Communication.
If you are from ST category then you can get at 11% of jee advance.
You can check this article for more information.
https://engineering.careers360.com/articles/iist-btech-cutoff