Despite injuries, mechanical gremlins, and evil willow thickets we've managed to come abreast of the Sentinel Tree. Also realized that the Secret Garden and the Sentinel Tree were farther apart from each other than I'd originally illustrated. Oh well, it's all good now. We estimate that the trail is now somewhere between 2/3 & 3/4 of the way to the center marker.
So, it looks like we'll likely end up building the cabin somewhere between the Sentinel Tree and the Ridge, and leaving that acre or so towards the middle open for the raised bed gardens or fall pasture (as it should still be green when everything else is bone dry). There are two silver-linings to this discovery: 1) very likely to be a decent well site near the ridge if the creek just went underground; and 2) we won't have to cut the trail quite as far before we start building :-D.