The vector field that defines the motion is the sum of a horizontal displacement of the columns, a vertical displacement of the rows, and a horizontal and vertical skewing.
The center of motion in this case will be where the displacements net zero, plus half the grid size, but you forget that you have a wider screen, so if you change the terms x and x into (x3) and x3, or maybe (x4) and x4, for your source coordinates, then you should get the center of motion close to the center of your screen.
