Generally when we speak of "film speeds" we are talking about exposure index, the only people who can say film speed is the manufacturer after testing according to ISO standards. None of us do that.
No safety factor, the reason I double the speed is because of the exposure scale of the paper, not to give me a "safety factor".
I thought that is what I had said......
I can't see you actually printing the shadow up around Zone IV. IMO, simply changing the EI doesn't really change the Zone placement. The relationship between the tones don't change. They are simply pushed up the curve.
I know I have not been doing anything wrong, I have been doing it for years.