Marty, All the math seems to be in order, until I post helix photos tonight, check the thread "Benchwork 101" in general talk. Two photos in my post there are relevant to your question about supporting the upper level. Photo 3 shows the 2x4 studs which are toenailed into a 2x4 fastened to the concrete floor. You have to take my word for that last part as you can't see the floor for all the crap stored under there. Is there anyone in the area who would like a couple 8' flourescent fixtures? Anyway, it kind of looks like the studs support the l girder, but they are actually about a foot and a half inside the girder. They are attached to joists which span the girders however, for lateral strength. In this photo you can also see the lowest level which is staging for the NYC, which is modeled on the main lower level, just above. You can see a portion of the unpainted backdrop for that level and a bend in that backdrop, the helix is behind that. You can see a steel u channel which is part of the upper level, at the top of the helix. In photo 5, you see the header across the 2x4 studs, with one section of plywood removed to show how I used the u channel inserted in routed out 2x4 pieces flush with the header to support the top deck. Lighting for the main level will be installed beneath the top deck and a valance added. In this photo you can see the fabric covered plywood I used for the backdrop, attached to the same studs supporting the top. And a portion of the main level covered with kit boxes and other misc. debris. All of which will be cleaned off in the next day or two, I swear.
My helix is on a five foot wide peninsula where the main level is divided somewhat down the center by the wall supporting the top. The top is open across so is viewable from both sides. It contains the main yard and engine facilities for the John Galt Line. Well, the yard so far, engine facilities to come where that missing section goes.
If your helix will be at the end of a stretch of benchwork along a wall, then yes, I would cantilever the top level from the wall.
Gary