I think it would help to explain there's no set Navigation DC and that the GM should use some best practices for how to make that determination.
Viewing post in Lost in Mortsafe Mire jam comments
You should reference the rules you are using when you change them. Especially when there are 2 sets of overland travel rules. One in the core book & one in the new material from the Western Reaches. The navigation rules do not set a DC which is why they are being expanded upon in Western Reaches, because it's unclear how it works in the core book. It doesn't account for the weather, familiarity, nothing. The DC check for it would be based on the standard 9, 12, 15, 18, 20. These checks are based on a natural 1d20 roll. You modified the roll, but not the DC after describing it as an "inscrutable labyrinth" that "proves... difficult." The question I'm raising is how difficult is it? Is it Easy on a sunny day? Is it Hard? How does the rain affect it? Does this change with the new rules? A little clarity would go a long way to making it more usable. If there had been standard DCs elsewhere, it would have been easier to determine what you think the environment difficulty for your adventure was.