If the trapezoid image is on the back side, change the back side.
If the trapezoid image is on the front side, change the front side.
Measure the distance between the bottom-left corner and the bottom-right corner. b in the figure above.
E.g. the distance between the corners is 190.8 mm (7.51").
Measure the distance between the top-left corner and the top-right corner. a in the figure above.
E.g. the distance between the corners is 191.3 mm (7.53").
Determine the difference between the distance measured at the bottom and the distance measured at the top. b - a.
In the example the measured difference is -0.5 mm (-0.02").
If the media registration is not optimal, change the value or use another setting depending on how the registration looks.