Viewport question with 3840x2160 monitor.
 

I just upgraded my monitor and will be using the native resolution of 3840x2160, but I'll be playing P99 in 1920x1200 mode.
I recall when Green launched there was a thread about the viewport command, but I cannot locate it now for the life of me. What do I need to do to have the game centered on my screen with the black bars on either side?

Thank you!