Loading 120 film on Horseman roll back

Hello!

I'm starting to learn about my "new" Horseman 980.
Now it's time to load some film and shoot; I have a 6x9 120 roll back
(Horseman original).
I figured how to load the spools, but I just can't figure how much I
should advance the film before closing the back... on my MF gear there
are dots that have to be aligned with the arrows on the film paper
back, but I can't see any similar reference on this film back.

Any advice?

Thank you! :-)

Fernando