Hi Rico,
re "proportional" of bellows extension to focal length, possibly a better choice of words would have been "positively correlated", though I still think the "proportional" holds for infinity focus. Your point about more complex designs (floating elements, etc.) is well taken, and interesting phenomena will certainly occur when you combine bellows extension and barrel extension with particular lenses. The overall point still stands as a first approximation and is confirmed by your own measurements.
I have not plotted any bellows/barrel extension vs. magnification or working distance. The only thing I ever did a bit more consciencously is the working distance for the MP100-stack-50/1.4 combination; it is roughly 35 mm, decreases a bit with closer focus of the MP100, and the focus of the 50/1.4 has hardly any effect, except for providing a "hood". With the 100/50 stack I can get up to 3:1 with a WD of 35 mm. It's ok for ladybugs or jumping spiders, but it gets pretty cr&ed in front of the lens. I can also get 3:1 with bellows, extension tubes and MP100 and have much better WD with plenty of space to put flashes, reflector card and all. However, it is a monster to handle, particularly in the field. I recently did some small clover flowers that way: what a pain (though good results in the end). The half-moons [don't recall what the proper name is in English for Schnittbildindikator] in the viewfinder are also becomming black as the effective f-stop is 4 x 2.8 = 11.2; (4 = magnification + 1). Should get a clear screen.
Re use of perspective control, I just got the bellows a month ago (used OM bellows w/o any perspective adjustments for many years). I tried the perspective adjustments a bit at home, and noted serious blurring with tilt. Using a small working aperture, the blurring seems to get reduced, though I have not shot any film on it. The blurring makes sense, as the periphery of the lens is used where various image degradations are more prevalent; and the improvement with stopping down also makes sense, as the contribution of peripheral rays is restricted. Has anyone some experience how much tilt still produces acceptable images at what f-stops?
Re using the bellows as a cheap alternatives for architectural photos, I had also wondered about that, but it will certainly not do. Here in particular, because of the short focal length of wide angle lenses; even a short bellows extension may not be capable to produce any image at all. The D21 with 13 mm tube has a WD of maybe 2 mm, and cannot produce any image mounted on a bellows. Bummer! So, if you are serious about architecture, get that monorail. Have been toying with that idea myself; Toyo has some entry level systems for about a grand that don't look too shabby. Monorail systems have the advantage that you can swap elements between systems much more easily. So there is no need to start with a Sinar.
Thanks for the clarification on SP/MP60.
Daniel