If you haven't done so already, download the Adventure Lab App onto your smart phone.
Once downloaded, browse for the "Commemorations in Newport" Adventure.
The adventure consists of five locations. You will have to complete all of them to get a full set of coordinates and find this cache. However, you can visit the locations in any order you choose. You may find it helpful to know that Stage 5 is closest to the bonus cache location with a walk of around 10 minutes to the cache location.
The bonus information you collect should be used in this formula:
N51 DA.(A-C-E)CB W002 A(A+C).(A-C)(B-E)(D-E)