Dell S718QL User Manual page 16

Check that the left and right edges are parallel to the screen. If you are
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unable to align the sides, skip this step and correct this with 4-Corner
adjustment on Page 27 Adjusting Image Corners.
Ensure that the rectangle is in the center of the screen (equal distance from
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left and right sides of the screen).
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Adjusting Projector Image
B
B
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D
C
C
D
D