Hey there
For CBSE you can use the formula that is multiply the CGPA with 9.5 to get the percentage. But full marks should be 100 for English.
Check out the link
https://www.google.com/url?sa=t&source=web&rct=j&url=https://engineering.careers360.com/articles/wbjee/amp&ved=2ahUKEwiqvIj89KfrAhWfyDgGHeLTBMUQFjAAegQIAxAB&usg=AOvVaw2uKnCPivLy7WtjEQunzbmP&cf=1
Good luck
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