Intriguing puzzle!
Based on my interpretation of the text, I believe the stream and overhanging rock are located here, where the AMR's property line, the stream, and the Gill Brook Trail intersect.
http://caltopo.com/map.html#ll=44.11...316&z=19&b=mbt
The text indicates a 30 minute walk
past this point brings you to a side-trail on the *left* that leads, another 5 minutes, to Wizard's Washbowl. If I understand this correctly, it means the washbowl is located east of the trail and in Gill Brook (given that 5 minutes walking-time is a short distance).
Another half-hour's walk brings you to side trails for "High Falls" and "Fairy Ladder Falls" and the trail to Nippletop. So Wizard's Washbowl is located in Gill Brook, on State land, about halfway between the AMR property line and the existing trail junction (Nippletop/Colvin). In other words, a bit south of the third designated campsite along the Gill Brook Trail.
http://caltopo.com/map.html#ll=44.11...1164&z=16&b=oo