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Drag the four yellow dots onto your wall's corners: 1 top-left, 2 top-right, 3 bottom-right, 4 bottom-left. Click a dot, then use arrow keys to nudge (Shift = 10 px).
The cyan box is the area being mapped. Grid lines are 12″ (or 25 cm) apart, so hold a tape measure against them to check your scale.
Measure the taped area from the wall's top-left corner.