So it is not okay to score with the back of an X-Acto after all. The paper is thicker, but it is still way too easy to weaken the fold too much by using an X-Acto. Like what I had been told when I learned to make plaster moulds, “never use a metal tool”.
A wooden tool seems to show some promise. It’s scoring too little, but I’d rather that it scored too little than too much. It looks like I can be more precise too, which is a good thing, I guess.