This cache is the 29th point on the Dartmoor Forest boundary, as established by the 1240 Perambulation. More details about the Perambulation can be found on the cache page for the Dartmoor Forest Bonus. On the front page of the logbook is an adjustment to be made to the Bonus co-ordinates, which you will need to find the Bonus.
The Rattlebrook starts off its life in the usual miry wastes of the upper moor, nestled between Hunt Tor and Kitty Tor. The disused railway line leading here originally served the nearby peat works, long since abandoned, but you can still see the rows where peat cutting took place.
The next point on the Perambulation is Stenga Tor. Head north from this cache to the head of the mire (waypoint below), where you can pick up a track that avoids the mire itself. This path curves south towards the military bunker on Kitty Tor. You can follow this path some of the way until you catch sight of High Willhays, when you will need to head cross-country to Stenga Tor.