some data
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With both base Zeefaring 6 and 7, the difficulty seems to be 253. With one +1 Zeefaring item, the difficulty seems to be 160 when base Zeefaring is 6 and 161 when base Zeefaring is 7 (at Persuasive 233, it's 87% with base Zeefaring 6, and 86% with base Zeefaring 7). Also, the number and kind of +Zeefaring items doesn't seem to make a difference.
So, based on this, adding any amount of +Zeefaring seems to reduce the difficulty by at least 92.
