I can add that later, but it's stated plainly in the RAW under "Navigation" on page 90 based on terrain familiarity which would vary. And, like all checks, "the GM chooses...a number called a difficulty class (DC) (p.9). I was careful to change only the roll; nothing else.
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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.