Thanks for the questions David. I'm going to suppose that you meant that the bottom of the seat is 10 sun from the floor (you wrote "1 sun") and the stretchers are 4 sun.
You are right that the stretchers have a 0.862043/10 angle for top and bottom shoulder cuts. The side cuts are chuh-koh koh-bai, which is 2.8734/10 in this case.
The single most critical thing in getting the sawhorse/stool to go together well is to get the backing cuts on the posts/legs just dead nuts on the money. If you do that, then the rest is relatively trouble free. I suggest developing the backing cut drawing on a sheet of paper such that you can expand the post section size to 3-5x larger than actual. That way you can set a transfer bevel or bevel gage to the drawing directly and accurately. A good choice for processing the backing cuts is to use a purpose-built sled in a stationary planer.
You are right that some of the angles get a bit fine and it can be easy to allow small errors to creep in. This is a tough project to pull off cleanly! The tan-gen shoulder cuts on the stretchers are not so bad - the tough ones to execute accurately are the cuts for top and bottom of the mortises for the stretchers, which are, in this case, sloped 0.2372 in 10.
Anyway, getting the lengths of the stretchers is easy, because once the posts are backed the inside plane of each post is square to its neighbor. Since the stretchers - nuki - are in plane with the common slope, the length is figured using the average slope only. That is, take the distance of the top outer arris of the nuki down from the bottom of the seat - seems like it is 6 sun in this example, in elevation view, and perfom a simple calculation of the length of hypotenuse, starting with the distance between post insides where they meet the underside of the seat. For example, if the top arris of the stretcher is 6 sun below the top, with a slope of 3/10, and the legs meet the top with a 10 sun space between then the amount to add to each end of the stretcher is 6 x 0.3 = 1.8. A total length for the nuki, along the top outer arris then is 10 + (1.8 x 2) = 13.6.
Anyway, I hope my clumsy explanation hasn't puzzled you too much. The article I am working on for the TF Journal right now is on the stool/sawhorse form, and will detail all the steps. Ya just gotta wait until March or so. Of course, in the meantime, fire away.
By the way, I'm back up in B.C. for the next three moths, teaching timber framing at the College of the Rockies. From what I gather, New England had had a fairly mild winter.
Chris
