Once I've found the nodal point for a given zoom lens and focal length, are there any consistent guidelines for finding the nodal point for a different focal length for that lens. For example, will an increase from focal length 24 to 50mm always lead to moving the camera forward or backwards? I found after finding the nodal point for 28mm focal length (35mm equivalent), I needed to move the camera forward for 42mm. This actually surprised me, since I was expecting I would have to move the camera backwards on the rail. At 120mm would it then logically be forward even more?