User blog comment:Kaido King of the Beasts/Chapter 818: Stepping Stones to One Piece/@comment-24442815-20160303163252/@comment-181.230.122.206-20160304023019

Maybe it leads out of the Grand Line, the 4 Blues, where the maps are usable, and then you can calculate a specific point out of that.

I bet that Nami's been working on a World Map since her journey began, and I also think that she has maps of the 4 blues. I think that maps of the 4 blues are common.

Regarding those of you who don't understand why Raftel is the final Island if there are two more after it. It's the final Island of the Second Half of the Grand Line, not the final Island of the World. That's one proof about what I've said above.