Your JEE Mains percentile roughly translates to an OBC rank of 30,000. If you maintain this rank in Advanced too, it is less likely to get an IIT. However,  I have seen many students improving their ranks from Mains to Advanced, so it is very much possible! There are 23 IITs, each with 5-6 branches, and therefore the cutoff range is very wide, therefore I suggest you to refer the following link to estimate the rank you would need to get a particular branch at a particular IIT.

https://engineering.careers360.com/articles/jee-advanced-cutoff

Coming to your question of how reservation can help you, a separate rank list is prepared for OBC candidates. According to the generally seen trend, OBC rank is nearly one-third of your general rank.