ScreenToWorldPoint won't return a constant value

Hi! I'm having a terrible time trying to get unity to properly convert coordinates. I just want to take a constant vector (73.9,40,46.2) and translate it into world coordinates. But even though the screen vector is a constant the converted world vector changes! Here's a snippet of code:

 Vector3 cameraPosition=Camera.main.transform.position;
 Vector3 maxScreenVector;
 
 maxScreenVector.y = 40;
 maxScreenVector.z = 2.0f * maxScreenVector.y * Mathf.Tan(camera.fieldOfView * 0.5f * Mathf.Deg2Rad);
 maxScreenVector.x = maxScreenVector.z * camera.aspect;
 
 Vector3 maxScreenToWorld = Camera.main.ScreenToWorldPoint (new Vector3(maxScreenVector.x,maxScreenVector.z,cameraPosition.y)); 

Here, maxScreenVector is a vector3 constant using screen coordinates (y is the camera depth, while x & z corresponds to those axis). I've also tried substituting maxScreenVector.y for cameraPosition.y, but it doesn't help. Do you have any ideas?