Openings, and what still comes off the roll
Moving a window changes the picture, not the number of strips.
Wallpaper is usually cut as full-height strips, then the window is trimmed out. A 120×140 cm opening is 16,800 cm² of paper you do not see, but those strips were still pulled from the roll. The planner counts that area and does not pretend you can buy less paper unless a strip is entirely outside the wall.
Drag the opening on windows. If you slide it across a seam, the seams you must cut around change. The strip count stays 8 for a 400 cm wall and a 53 cm roll, because the width did not change.
Measure the opening on the wall, including the frame you will not paper. Type centimetres or switch to inches. The internal math stays in centimetres.