It works as written unless you house-rule it, which admittedly is not very well. I house-ruled Assursnce, and I also house-ruled the listed simple DCs because I felt they were too harsh for the game I wanted to run.
You can see my house rules here. I picked assurance values equal to one less than the simple DC for that proficiency because it made them roughly track when you would start succeeding using the core rules.