It's actually a simple vector equation once you have the position and direction vector. The position vector should be the camera position and the direction vector is the vector returned by the reproject...WS-method. If y of the target plane is known, you end up with three equations and two unknown variables x and z...that's pretty easy to solve on paper. I have some code lying around that does this iirc...i just can't post it right now, because i'm on holiday. Maybe wednesday if you still need it then.