Since the white line is so thick, do I need to use a different margin? Or could there be some laminar wedge present?
I think *generally* the breakover & edges of the hoof walls(inc 'rolling') should be put where they would be if the foot was healthy. In a healthy foot, the white line is very thin & tightly attached. Therefore, assuming your horse should have 1/8" thick inner wall material on the ground, that + a few mm leeway is about how much should be on the ground from the *inside* of the white line.