You can get a position relative to the screen resolution, then it's just a matter of tweaking the values to find what works.
From http://kinectxna.blogspot.ie/2012/02/tutorial-5-skeletal-tracking.html :
foreach (Joint joint in skeleton.Joints)
Vector2 position = new Vector2((((0.5f * joint.Position.X) + 0.5f) * (resolution.X)), (((-0.5f * joint.Position.Y) + 0.5f) * (resolution.Y)));
spriteBatch.Draw(img, new Rectangle(Convert.ToInt32(position.X), Convert.ToInt32(position.Y), 10, 10), Color.White);
Instead of drawing to this position you could use it in your update logic. The values aren't perfect but they should get you on the right track.
If in fullscreen you'll want to find the current resolution of the user's monitor. You can find it using:
Just put this in place of resolution in the above code and you should be good to go.
If this doesn't work try:
Where graphics is the GraphicsDeviceManager (declared in the game's constructor when creating a new XNA game). Depending on the scaling of the game one or the other of these should work.