You can hold the plexiglass guide up to the hoof's side, sliding it backwards or forwards until you can line up the 30 degree hairline slope against your horse's hairline. This will help you sight your horse's hairline. If it is 30 degrees, then you should have the bottoms of the coffin bones ground-parallel. With a rotated hoof, you will want to back up the point of breakover to where this line hits the ground. Then you would slide the plexiglass gauge backwards until the 55 degree line lines up with the top of this hind foot.
Any toe hitting the ground forward of where the guideline hits the ground needs to be backed up.
As this hoof has some dishing, it does need to be backed up. For foundered horses, you back the toe profile up more aggressively. For navicular horses, you would just do a mustang roll to the toe. The most common problems with navicular horses are high, folded over bars pushing up into the hoof capsule, putting pressure on the navicular area, high heels and heel contraction. Relieving these problems will often give surprisingly fast improvements.