At the top ( summit ) off hand I'd say 40 MPH , 4th gear stage 1 , if you find your building up too much speed; having to apply brakes too often ; go to stage 2, still too much speed , slow down with brakes go to 3rd stage1, then stage 2 if necessary.
My coach with only an exhaust brake handles that grade at 40 > 45 with very little service brake application. Exhaust brake and your stage 1 give about the same hold back.
I only drove a 2 stage equiped unit a short distance , isn't the down shift pre -programed into the engine brake function? Do you have an owners manual ?