The best is to keep your phone at 40-70% but you might not enjoy your phone that way.
Charging it before it goes lower than 20% will do.
Or charge it before it goes lower than 30% if you can.
Turn off the phone at least once a weak for a few hours, make sure the battery is partially charged when you turn it off.
And don't worry about calibration it simply takes time for the battery reading to be calibrated when you install/update a new ROM.