There are 360 seats for Arts and Science stream at St. Xaviers. There is 15% of quota for Management seats, 50% seats are allotted for Christian Minority and 42% seats are allotted for General Category Candidates. 5% is for PWD Candidates and 3% is for Special Category Candidates. Now  under Christian Minority quota, preference is given it its in house Candidates. So you haven't studied your 12th in St. Xavier's you have to apply under General Category. There are approximately 11 seats allotted for General Category. So if you have to secure a seat under this Category, your 12th aggregate should be more than 90%.