if i do previous years jee mains paper and understanding the logic behind each ques...how much will i get?

Hii Samarth,
First of all your all doubts should be clear about any subject or concept then only you should solve the papers.As per your question it is good to solve previous years questions ,it will give you a idea of the questions that will be given in the exams and sometimes the questions are repeated just the values are changed.You can easily score above 170.But this will be effective only your concepts are clear.