D-U-R-X advice on the current widget is great. Once every month or two, let your phone die, power on again and wait for it to die a second time.
Charge till full, using the current widget to let you know it is full. For the next few weeks your battery will report the correct level.
Do not do this to often. Normally best to recharge at 15-30% and pulling from the charger when full. No harm pulling it even if not full.
Draining the battery completely, and charging the battery completely, too often stresses the battery and shortens its life. By design most devices report full at 90-95% and empty at 1-3%.