Hello, 

The expected cut off for SC category is 31-35 marks in JEE MAINS 2019. 

Your percentile score is determined by the performance of other students who appeared for the exam. It tells how many students are equal or below you in any particular exam. Rank will be dependent on this percentile and will be used for admissions in different NIT's, IIIT's, and other state Colleges. 

You can predict your rank by the given careers360 link using your JEE MAIN application number, score and category.

https://engineering.careers360.com/jee-main-rank-predictor?icn=JEE_Main_rp&ici=JEE_Main_article_view_sticky

Hope this information was helpful. 

Good luck!