I think you mean in relation to where the camera is.
Okay, XZ plane that goes through the origin of the world at (0,0,0), right?
I’d use gluProject to build a ray and intersect that against your XZ plane. That will give you the 3D position on the plane.
gluUnProject(winX, winY, .3,
object1X, object1Y, object1Z);
gluUnProject(winX, winY, .6,
object2X, object2Y, object2Z);
Make a vector from the first point to the second point. This will be in world space. Normalize it and you have the direction for a ray. Your ray origin is your camera position.
Do a ray-plane intersection that like the one below. Move the ray t units in the direction of the ray and you have your plane intersection!