Rokkenjima is described as heavily forested. It looks similar to Nijima in climate, which is also heavily forested. It also takes about 30 minutes by boat to get there, and the harbor in Nijima is on the west side of the island, so if Rokkenjima's off to the southeast somewhere you're probably eating up 3-5 minutes just getting around the southern tip of Nijima.
So it can't be very far from Nijima at all.
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