The best way to keep your battery calibrated is to only plug it in when it goes under 20% and then to charge it completely to 100%, without interruption.
If you keep doing "shallow charges", ie. charging your phone when it's like at 50% or higher and then unplugging it before it hits 100%, the charge meter will become slowly inaccurate over time.
As mentioned above, best way to recalibrate it is to let it run out completely and then to charge it overnight to 100% and just leave it charging for a couple hours.
The phone will automatically activate "trickle charging" when it get's to 100% so it never actually overcharges your battery anyway.