Here is an app note from Maxim describing how to implement touch screen calibration. It provides the reader with the necessary mathematical tools needed to address issues with mechanical misalignment and scaling due to imperfections in the touch screen:
Mechanical misalignment and scaling factors can lead to a mismatch between the values from a touch-panel system (in this tutorial, the touch-panel system implies a setup comprised of a touch screen and a touch-screen controller) and the display (typically an LCD) on which the touch screen is mounted. This article discusses the mathematical techniques to calibrate the touch-panel system so you can match the graphics on the display to the output from the touch-screen controller.
Why haven’t I had to calibrate my Kindle Fire?