Your cartridge is attached to a flat plate. That is what should be parallel to the disk as a starting point.
Usually, but not always, that plate is parallel to a a linear extension of a line bisecting the upper and lower lines of your conical arm. You can often see that line by shining a bright flashlight on the arm from the side.
Usually, but not always, that plate is parallel to a a linear extension of a line bisecting the upper and lower lines of your conical arm. You can often see that line by shining a bright flashlight on the arm from the side.