What is disclosed is an apparatus and method for calibrating an optical module of a document scanning device. In one embodiment, imaging sensors of an optical module of the scanning device are used to obtain an image scan of a calibration strip mounted on a surface of a platen on which a sheet to be scanned has been manually placed. The calibration strip has a first vertical line at a first distance from a right edge of the strip, a second vertical line at a second distance from a left edge of the strip, and a sloping horizontal line positioned between the first and second vertical lines. A slope of the horizontal line is non-zero. In a manner more fully disclosed herein, the image scan of the calibration strip is to obtain calibration measurements. The calibration measurements are, in turn, used to calibrate the scan head of the optical module.