Well it's not entirely about DoF. The idea is to find the aperture that will give you the sharpest image for the depth of field required. It's a trade-off between a small enough aperture to create the DoF you want and the diffraction caused by stopping down too far. Since I saw this my taking technique has been to find the bellows extension difference between near and far and then consult the chart for the "best" aperture. My shutter speed falls out of all that. Kinda like Large Format Aperture Priority!!!!