To my knowledge Apple has not provided any type of calibration function for the digitizer. The only thing that I can suggest is that you perform a hard reset by holding the power/lock button and the home button at the same time until the unit shuts down and then, if it doesn't restart on its own, power it back on with the power/lock button only (you don't want to accidentally put it into DFU mode unless it's ok for it to be completely wiped). If that doesn't work, the only suggestion that I have is to try another digitizer. Unfortunately, if that also doesn't work, then you are looking at a board level problem which would likely require micro soldering to repair.
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