Calkins Brook Truck Road is the major dirt road that crosses the Blueberry Trail. There are signs marking the intersection, and a sign a short distance on the left warns against passage that way as it is private property. You want to take the road to the right, south, toward Calkins Brook lean-tos. If memory serves, this intersection is 1.2 miles from the parking area, then you have to follow the truck road another 2.4 miles to the herd path.
The Calkins Brook herd path branches left at a hard right turn of the truck road. It's just after a fairly steep descent, and, as noted, there's (probably) a cairn marking it. Within five minutes the herd path will bring you to Calkins Brook, which you cross. It may appear that the path stays on the left side of the brook, but it doesn't.
It's pretty easy going until just after the last water crossing, which I think is at about 3500 feet elevation. There is a section of blowdown almost immediately on the other side of the stream, and, while the path is clear, it will slow you down. (Incidentally, watch at this crossing on your way back down. I've done it twice and both times missed the path somehow and went off to the right and thrashed around a bit before retracing my steps and relocating the path.)
Not long after you clear the last of the blowdown , you reach the ridge, about 4000 feet. The summit of Donaldson is a short distance up to the right, on a big rock on the left side of the path. The summit sign was gone last September, but there was a red trail marker high up in a tree. Nice view up to Seward and across to Seymour.
