Once you've created the HoloPlay Preview settings you can check "Manual Position" and then modify the X and Y position. 0,0 is the bottom left of your main monitor. If your Looking Glass display is located to the right of an HD (1920x1080) monitor then the X and Y position would be 1920, 0. Since you're shifting it 1920 pixels to the right, and 0 pixels up or down.
If your monitor is to the left of your main monitor, the X offset would be negative resolution of the looking glass display. For a Portrait, that would be -1536.