Your GaugePilot makes calibration to the roadbook mile/km incredibly easy.
Firstly, you can change between Miles and KM as your distance units and your unit will remain calibrated. No need to change gears or to change your calibration value between events using Miles and events using KM - just tell the unit which units you are using and it does the rest.
Calibration itself is straightforward. Start the calibration application, drive the calibration distance, adjust the odometer to match what the rally organiser states that distance is, and press store. That is it!