As I tried to say before the ray doesn't care where the edge of the viewport is.
Furthermore you can't get that position without some actual raycasting, and the viewport offers nothing to cast against, unless you make some dummies.
However, you can presume that the layout width divided by two would be long enough to ensure that it will always end off screen.
Perhaps you could describe exactly what you want to do. Step by step.
You keep asking for an angle, and I have no other frame of reference other that those I've already addressed.