I must admit to being slow to recognise the significance of this map, I saw it a few days ago from a comment by Tonzi in the Cipher Explanations page and didn't realise what it was.If you haven't finished the book yet you may not want to read on.
Then last night @MyTheseus on Twitter pointed out a new blog post on WhoIsStraka.WordPress.com which had the same conclusion, although arrived at via a different thought process.
Firstly the map is quite important as it explains the relationship of the coordinate system to the codes used, they are linear as you can see and these aren't locations from the book, these are all coordinates that can be found when LONOE is in the wheels window.
When you look at the map you can see that Maraú in Brazil (blue pin) is close to this line, on the postcard dated the 20th of April (insert on page 201) Eric writes that he is near this place.

So the conclusion is that the missing letters L O N O E are actually a pointer to the location of FXC should Straka have ever picked up the message hidden in the footnote cipher.
Tonzi very kindly created this as a Google map for us all to use and see this linear relationship.






In the first footnote for Chapter 10 the words "Spinning Wheel" and "locate" are in italics, so this points us straight at the Eotvos wheel contained in the book.
If you work out the locations referenced from the footnotes and write down the 5 letter codes in a grid you get a message -
I HAVE LOVED YOU FROM THE BEGINNING. I WILL LOVE YOU TO THE END.
There is still the question of the location of the 6th column, as the footnote mentions Calais but the coordinates for Calais France do not match the code necessary to make the whole string work fully - L O N L E
The list of locations is -
1 - Bergan, Norway - 60N, 5E - I Y B W U
2 - Biabou, St. Vincent - 13 N, 61 W - HOEIT
3 - Portland, Or - 45 N, 123 W - AUGLO
4 - Thunkar, Bhutan - 27 N, 91 E - VFILT
5 - Santiago, Spain - 33 S, 70 W - ERNLH
6 - Calais, France - 51 N, 0 E - XBTUP (??)
7 - Morondava, Madagascar - 20 S, 44 E - OMIVE
8 - Havana, Cuba - 23 N, 82 W - VTNEN
9 - Sofia, Bulgaria - 42.70 N, 23.33 E (approx) - E H G Y D
10 - New York City - 44 N, 73 W - DEIO*
You can see the discussion on the Eotvos wheel code page, Click here to see the first comment and then scroll down slightly to see the post by ObFuSc8