The screen color is probably the most important parameter and you should always pick the screen color before doing anything else. It should be set to the color of the green or blue curtain behind the foreground object. Pick the screen color directly from the image by Ctrl/Cmd+Shift+Alt dragging a rectangle over the blue pixels. The average value of the pixels selected is used.
TIP: If you press Alt when sampling a color, Nuke always samples the source image regardless of what you’re looking at. This means that you can pick the blue screen color even if you are viewing the matte, status, or composite.
TIP: Picking different shades of blue from the screen color can give quite different results. It’s worth picking from different parts of the screen to get the best result.
Picking the Screen Color creates the screen matte used to composite the foreground over the background. It also sets the Screen Balance (if this has not already been set manually) and despills the foreground.