I am having a problem calibrating the touch screen on the 4D Systems Gen4 7" LCD display with a resistive touchscreen. Near the center of the display the calibration is very close but near the edges it’s off by more than 0.25 inch. At the lower left corner of the LCD the pointer appears approximately 0.25" above and 0.25" to the right the touch point. At the upper right hand corner the pointer appears 0.4" below and .3" to the left of the touch point.
The instructions for calibrating the touchscreen in the Gen4 user manual do not seem to apply. My environment is a BeagleBone Black with the latest image of Debian Stretch (Ver 9.5 dated 2018-09-16)
How can I calibrate the touch screen?