DanK helpfully explained the following to me in Brian's thread:
a very rough rule of thumb. As a starting point, assuming the lens can reach 1:1 without extension, assume that the maximum magnification will be somewhat greater than (FL+EL)/FL. So, for example, a full 68mm of Kenko tubes on a 90mm lens would come out to a maximum magnification somewhat greater than (90+68)/90=1.76:1. I think if you tested it, you would end up with something closer to 2:1. From this you can see that the shorter the focal length, the greater the magnification.
I did not realize this. Now that I do, I wonder if my ultimate choice of a macro lens with a 1:1 magnification if used with extension tubes shouldn't be with a shorter focal length of 40mm or 60mm rather than the longer end of 85mm to 105mm. That's mostly because of reduced cost assuming all other factors are reasonably the same. Are they the same?
As an example, the Nikon 40mm is a DX lens (I never plan to shoot FX cameras) that costs only $250. Its shortest focusing distance is 6.4". I'll always be working in my makeshift studio. I won't be shooting live bugs so I don't need a longer focusing distance in that regard. Will it help to have a longer distance of about 9" or 12" provided by a 60mm or 85mm lens, respectively, to allow more lighting flexibility? I'll always use continuous light and/or off-camera flash units.
Anything else I don't know to ask about?