How do I convert CGPA to percentage in IIEST, Shibpur?

You can’t. There is no direct formula for converting CGPA to percentage. After the end of semesters, the mark sheet you will receive will contain your CGPA, the total marks you obtained with credits for each subject. Now CGPA is a function of both the marks you obtained in a subject and the credits it carries. While percentage is only a function of the marks you obtained in a subject. So, there is no formula to convert percentages to CGPA and, vice versa. But simply multiplying your CGPA by 9.5 will give you an approximate answer. 

It will be better if you go through your mark sheet, and note down the marks and calculate by yourself.