The lower tension is correct , a 1 1/2 turn then upp to takeup spring, the photo angle not showing full turn. I threaded as Juki TSC-441 manual indicates, Section 12, page 6 of the attached manual.
Im not sure how much tension there should be in bobbin,any tips how you measure that?
I suspect i have to little tension on bobbin and i just use a 8oz leather, it might be better as u say with 2 pieces.(thicker).
Im trying your tips tomorrov,updates follows.