The ActivCalibrate app is pre-installed on the ActivConnect G, and can be used to calibrate your ActivPanel. 

Ensure the ActivPanel is connected to the USB port on the ActivConnect G.

Go to Settings and tap ActivCalibrate.
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The first time you open the app, you will be prompted to authorise ActivCalibrate to access your ActivPanel (USB device). Check the box, and tap OK.
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Touch the centre of each highlighted cross with your finger.
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After the fifth cross, the ActivCalibrate app will close automatically. Your ActivPanel is calibrated and ready for use.