What a battery calibration does is actually to just clear or delete the current calibration on your device. After that, it's up to android to recalibrate your battery based on your usage. The reason it need root access is because the calibration data is located in the area of your android that is protected.
If you really need to recalibrate, then you don't have other choice but to root and install those software, reset to factory default, or flash your software again.
Rooting is fairly safe as long you are aware what apps you are installing.
