DOF And The 45 mm

If that worked, life would be much easier, indeed. However, while the doubling of the Camera to Subject Distance appears to increase the DOF by the square of that increase (by a factor of 4), and the halving of the (linear dimensions) of the (effective) sensor-size (due to post-cropping) appears to (by reducing the Circle of Confusion diameter by a factor of 2) only reduce that factor of 4 increase in DOF to a factor of 2 increase in DOF ...

... you are forgetting that the image-enlargement (by a factor of 2) required in order to scale the displayed image back to the same physical viewing-size (also) reduces the Circle of Confusion diameter by a factor of 2. The result the above described effects combined results in no net increase in DOF.

See "Image Enlargement", Page 4: http://www.largeformatphotography.info/articles/DoFinDepth.pdf

A doubling of the lens-system Focal Length completely cancels the effects of the doubling of the Camera to Subject Distance. It is the doubling of the F-Number that doubles the DOF in the above case. The same doubling of DOF could be achieved with a 50mm Focal Length at F=3.6 without doubling the distance.