As the paper pattern of Jee advanced keeps on changing so it is quite difficult to provide the marks but it would be better to get more than 60% if you wish to get admission in top IITs.

Hope this helps!!!

Yes, there will be chances that you will get IIT Delhi chemical branch if you get a rank around 1200.

IIT Delhi rank closes at 2996 in general category.

You can also check the cut off rank from the given below link

IIT Delhi cut off

Good Luck!!