I start with the Impact Plane. It should lie in the intended horizontal direction.
Its vertical angle should:
* Be flat enough to prevent erecting of the posture through the Impact Zone
* Is almost always more vertical than the Address Plane
* Will usually run through the core of the torso, and
* Is of little importance, in the sense that, a plane with any incline angle can have the intended horizontal direction.
Many of the most consistently accurate players of all time have, from a point late in takeaway to the beginning of the Impact Zone, kept the shaft parallel to the intended Impact Plane with the left arm parallel to that plane at the top (Tiger Woods circa 2000). This mechanical feature of the stroke is least likely to cause the Impact Plane to vary from intended, since there is no required change to the vertical angle of the shaft approaching the Impact Zone.