Hi Ashish, 

The criteria for calculating top 20 percentile for B.tech admissions is as follows:

 For B.E./B.Tech. programmes

 i. Physics

ii. Mathematics

iii. Any one of Chemistry, biology, biotechnology, technical vocation subject.

iv. A language (if the candidate has taken more than one language, then the language with the higher marks will be considered)

v. Any subject other than the above four (the subject with the highest marks will be considered).

So there is no problem with computer science being your optional( as it has higher marks as compared to physical education)

Thanks.

no, in future there will be no problem as all companies, exam eligibility take the percentage as best of 5 . 

so , your best 5 subjects will be--

physics  --- 92

chemistry --- 95

maths---- 95

english----74

computer science----98

total percentage ( best of 5)----90.8%

by the way, your percentage is quite high