From a Dahon, to a Birdy, and finally through a Brompton list I follow comes the following tip that seems to work spookily well. See, my wife and I share a few bikes, and as I’m taller we have to re-set the saddle heights several times a week. Usually we eyeball it well enough, but too often not well enough. This should fix that:
A local frame builder (who is also a veteran racer) once showed me how the European race teams deal with quick fitting bikes in case of a mechanical problem — It seems that for most or all folks, the distance from the arm pit to the tip of the index finger is the same as the correct distance from the top of the saddle to the center of the bottom bracket. Now I know there may be slight errors, especially since the crank arm length can vary from 165 to 175 (1/2 inch), but I have tried this myself, and it works. What you do is put the seat in the crotch of your arm pit, and adjust the seat post until your finger tip comfortably rests in the middle of the crank bolt. It may not be perfect, but it will be within 1/2 an inch, and probably more like within 1/4 of an inch.
I can probably forget now that my saddles should be 77cm from BB center. They should be “arm’s length.” It has that primal, biblical, cubit-and-span-and-the-king’s-foot thing going on. Does it work for you?