When left clicking to position the 3D Cursor in 3D View, when in User Perspective view or User Ortho view, and more precisely when there's no object under the mouse, the position of the 3D Cursor looks random. I know it's not, there must be a way Blender determines it's location, but I can't find that out, is it a specified distance from view? In case it is, what is it, and can this distance be changed and manipulated?
The 3D cursor moves in the plane facing the view you are in and intersecting its previous position.
Is you set up this configuration : a camera (bottom right), a plane (selected) 90° intersecting the camera view, you can notice this behaviour. Go in the camera view using 0, click somewhere to move the 3D cursor, and come back to the top view. The cursor will stay on the plane.
The distance of the plane is defined by the normal vector N of the view (N normalized) dot the camera-"prev cursor position" vector. Or if C is the camera, P the previous position, the distance is ||N.CP||