Requirements root permissions!
Calibration needs to be done after flashing a new ROM, but you can calibrate any time you think your battery is miscalibrated. This program does it by removing the batterystats.bin system file. The OS generates a new clean batterystats file soon, thus any fake information from the previous ROM is removed.
It's suggested, but not necessary, to let the phone fully discharge after calibration, then charged to 100% without break.
The app shows your actual battery status, and only allows you to start the calibration (i.e. removes the file) when it's fully charged, unless you force earlier calibration.
The voltage meter can be useful too. Full charge is around 4200mV.
There is an option to beep when 100% is reached so you don't have to closley follow the changes.