The Cache is not at the posted co-ords and is not necessarily near the end of the walk either.
This bonus cache can be found by finding clues in six of the larger on the CHATAW series and working out the location from the formula: N 53 53. (A-D) D (B/C) W 000 11. ([E x (B+F)] - C).
Mystery caches usually involve cachers either solving some sort of puzzle to obtain the co-ordinates or collecting clues from other caches and the rating of the cache reflects all the difficulties you encountered to obtain the co-ordinates, be this solving puzzles or the difficulties of the caches that you needed to visit to get the clues. A top tip here is to solve as much as you can at home if there is a puzzle, and when collecting information from other caches, work out the final co-ordinates as soon as you think you have all the information. It may save some leg work later. 
(Note this hot tip has been added after the experiences of some of the first cachers on this series).
The question mark symbol can also denote that it is a Challenge cache where you need to complete a specified task before you can hunt for and claim a find on it. This could be fining a certain number of caches in one day, finding certain combinations of caches, finding 30 caches that you have previously logged a DNF on, etc, etc It's always worth looking at these caches as the challenges can set you new goals for caching days out.
The cache rating here reflects all difficulties and terrain encountered in obtaining the information needed to establish the co-ords as the hide itself is an easy find once at GZ.
When logging please feel free to share your own top tips and hints.
Congratulations to MGowners on their FTF on this cache and on 20 of the others in the series - getting up at 5.30am really is dedication to the cause!
Also congratulations to The Biffa's on reaching their 5,000th cache milestone at this cache!