[Q] Correct process for calibrating the battery?

Hi, My Micromax Canvas HD's battery was draining very quickly. So, I purchased a new battery but no success. I searched google and came to know that battery needs to be calibrated after flashing a new ROM and i also updated my canvas hd from 4.1.2 to 4.2.1 JB. For calibrating, device needs to be rooted. So, I rooted my device. Now, this is the point where i am confused. There is an app on playstore "Battery Calibration" by NéMa. I downloaded the app and followed all the instructions in the app about calibrating the battery. This app deletes batterystats.bin system file to calibrate the battery. After calibrating through this app, I saw some improvement in battery but i noticed that battery power goes down at normal speed upto 65% but after that, it goes down rapidly. What is the problem? If i am using wrong method to calibrate the battery, then please give me the correct & best one. Thanks in advance.