Perfectly centering a second monitor on the primary?

[RackMonkey]

My primary is a 3840 x 2160 TV with a 1920 x 1080 monitor underneath it. In order to set up some export windows I need to center the the second monitor at exactly 961 pixels from the left but when I try to center the second monitor on the primary with the mouse, Windows and nVidia both jump 34 pixels either side of the 961 point.

 

Is there a config file in Windows somewhere that I can manually set the position that I want?