I would say that you need to ideally target 90% to be assured of a seat in CSE in category 1. This is because there are about 400–500 seats in category 1 (min. fees + merit) and since the number of applicants will vary depending on expectations.
Earlier, there was a strong overlap in Advanced and Mains toppers with VITEEE toppers so the CSE seats were available upto 6000 VITEEE ranks but this time, I expect it to be very different. Most likely the competition will be between the top 1000–1500 applicants with the highest 12th std marks. Some of them will do well in IIT and Mains but the rest will opt for VIT, SRM, MIT or BIT.
Based on this, I feel 90% is a minimum for CSE. Having said that, you need to keep in mind that 12th std mark also includes state boards and Karnataka and Andhra board student will have a huge advantage as the top 20% scores average around 92–95%. So that is an uncertain factor for sure.