Let’s see if I follow, as you have left out a bunch of details…
Presumably you mean to create the line and plane as
line = [camera.eye, face.normal]
plane = face.plane
(since otherwise I don’t know what line and plane you are talking about). But then
returns a Point3d on the face’s plane where a ray line through the camera eye passes perpendicular through the plane. Would you then use that point for the subtraction I proposed? If so, please note that this point is on the face’s plane but is not necessarily within the face and could actually be behind the camera or otherwise outside the viewable area (technically, the camera’s frustum). If that’s not the idea, what would you do next?
Also, note that because the line was constructed using the face’s normal, the vector between the intersection point and the camera eye is necessarily either in the same or the opposite direction as the normal.