Start with a test strip
Print a long ruler segment first. Longer test strips make drift easier to measure and give a cleaner correction than a tiny sample.
Measure the output
Measure from the printed zero mark, not from the paper edge. Record the difference in inches or millimeters and enter it into the calibration tool.
Reprint and verify
After applying the correction, print again and check the new ruler length. Repeat only if the printer still misses the target.
Open the print ruler app to calibrate now.