Hello aspirant  

To score good percentage in 12th board exams, you have to work hard. Maths and chemistry are the most securing subjects. Also, 30 marks are from practical in chemistry and physics, you can score 30 out of 30 by complete knowledge of experiments performed in the lab. Also, mostly questions are from NCERT, do it seriously. Try to solve previous year exam problems. Only 10to 15% paper consists of new question, so you can easily score 80-85 but to score more than 90, practice a lot, revise again and again. Other subject is English, and you can easily score above 90, maximum marks in English are of writing section, practice the articles, letters etc. Go through the new and latest articles.

Hope it helped to clear your delimma.