How can I re-calibrate the touch screen?

You can force a re-calibration by removing the file /etc/pointercal and then rebooting the module. The calibration is then run during the module startup. It creates the file /etc/pointercal, and if this file exists at startup, the calibration step is not run.
You can also do a calibration manually by running these commands from the console (Advanced View in G2Link):

/etc/init.d/qml-viewer stop
/etc/init.d/qml-viewer start

Do I need to give the end-user of my equipment the option to re-calibrate the touch screen?

In the case of Projected Capacitance (glass front, PCAP) touch, the answer is generally no. Projected Capacitance touch works by energizing and sensing patterned rows and columns of lines in the touch panel. Calibration provides a correspondence between a location on the LCD and the touch’s sensed location. This won’t change over time, so one calibration should be sufficient. There could be a rare case such as where the calibration was done looking straight on, but the display is always viewed at an extreme angle; here the option for the user to re-calibrate might be helpful.

For resistive touch screens, the touch sensor surfaces don’t have discrete row and column lines, but still, the need for user re-calibration is not clear. Cars and other long-lived appliances use resistive touch screens and do not offer user-calibration. If there is a concern, then you can add the re-calibration option on a maintenance screen.