Try to not turn your hands off of their vertical to the ground alignment in the backstroke and let the gyroscopic motion of the clubhead turn your hands towards the plane for you. Don't actually try and turn the left hand towards the plane....
Use the pressure from extensor action towards the plane line. As you turn your shoulders your hands will automatically maintain their relationship with the plane line bringing the club up the inclined plane. Body turns and the arms swing upwards.....
So, in order to stay on plane, someone with less turn would need more "in" with the arms in order to compensate for what ever isn't provided by the pivot? Or, conversely, someone with a very large shoulder turn would need more "up" with their arms (relative to their body) in order to compensate for a big pivot rotation??
The incline plane is a product of two circles: one vertical- the arms, and one horizontal- the pivot. Together they form an incline plane attached to a base line or Plane line. The beauty is tate the angle of the incline can be anything you want. Homer said to take the club Up Back and In -- On Plane, “Whatever Plane you want to be on.”
Remember that great Airplane song White Rabbit-
“One circle makes you tall
And one circle makes you flat
And the ones that Leadbetter gives you
Don't do anything at all.
Go ask Homer
I think he’ll Know”